Heath Doorbell MOD

8/4/2019 Heath Doorbell MOD

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Heath / Zenith Doorbell Modification

Turning a Heath / Zenith Wireless Doorbell into a Remote Control Relay

Last night I made a pot of coffee, stuck my copy ofColossusinto the DVD, ( "Colossus" is in

my top-10 movies for hackers and geeks. Besides, Susan Clark is kind of sexy in a 1970's, geek-

chick sort of way) and set out reverse engineering the receiver. I have a method of reverse

engineering circuit boards involving digital pictures, Photoshop, overlays, and CorelDraw that is

probably suitable for an article all it's own. But for now suffice it to say that by the end of the

movie I had a schematic, a decent caffeine buzz, and I really had to get to the bathroom. If you

click on the circuit image at the left you'll open a new page with the entire schematic for the

doorbell.

I've said many times that I might be sixteen kinds of geek, but RF-geek isn't one of them.

Perhaps one of my radio-gifted readers will contribute a decent discussion of what's going on in

the left two-thirds of the schematic (and maybe even tackle the antenna question). Luckily for me

in the immediate project the RF stuff really doesn't matter. Looking at the schematic we can see

that the bulk of the parts count is for the RF section. The transistor in the upper right is the audioamplifier, and the COB is the brains of the thing.

(RANDOM NOISE: "COB" stands for Chip-On-Board. For a regular integrated circuit (IC), a tiny

piece of silicon is bonded to a lead frame and then encapsulated in plastic. The lead frame

ultimately becomes the ICs legs, and are soldered to traces on a circuit board. For COB, the tiny

piece of silicon is bonded right to the circuit board and traces, and then a blob of epoxy is

dropped over it to protect the die and die bonds. This saves time, space, and money for the

manufacturer and you'll see this a LOT in consumer stuff made in China. )

The address header allows the user to select one of 128 distinct addresses for the receiver and its

transmitter. This opens up the potential for some interesting remote control hacks but for now

here's what we need to play with:

What we have is a numbingly simple audio amplifier. It kind of surprised me that there's no

current limiting through the speaker at all but then it occurred to me that it might not be an 8-ohm

speaker. A quick check with the meter and sure enough, the speaker IS 8-ohms, so it's Ohms Law
http://www.imdb.com/title/tt0064177/http://www.imdb.com/title/tt0064177/http://www.imdb.com/title/tt0064177/http://www.imdb.com/title/tt0064177/


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time!. Current = Volts / Ohms, so 3 volts / 8 ohms = 0.375 amps, or 375ma. Power = amps X volts,

so 0.375 amps times 3 volts = 1.125 watts. Now, granted, the calculations are for DC current and

the speaker is seeing some kind of pulsed or modulated DC, driving that tiny speaker with a watt

seems a little close to the limit. Since they have been selling these things for a number of years

we'll presume it's not a design flaw, and just chalk it up to 'art'. Next we need to know what sort of

signals are going through this circuit. Using an oscilloscope I'll make measurements at the three

labeled points of the circuit. But for now all I have in front of me is a meter so let's check out the

power.

Power play

Brett wants to run this off of his PC power supply after all so we need to know how muchpower (current) the receiver draws. A long time ago I made a silly little widget to make this sort ofmeasurement a lot easier. It is a little piece of double-sided printed circuit board material aboutthe size of a dime. I soldered a piece of test lead wire to each side and soldered female bananajacks to the wires' ends. To measure the current draw of a battery powered device (like ourdoorbell) you simply slide the little PCB in between two of the batteries or between the end of onebattery and the spring. You then plug your current meter into the banana jacks and away you go.

(RANDOM NOISE: By the way, this is a handy way to add remote control or timers to batteryoperated devices that you do not want to physically modify. Using the same little PCB betweenthe batteries, use a relay to short the wires together to turn the device on or off. For example, turnoff the smoke detector while you're actively burning dinner, turn a boom-box into an alarm clock,turn on the lantern when you open the shed doors, etc.)

With the receiver sitting idle my meter indicated a current draw of 0.22 milliamps. Forcomparison consider that a regular old red LED draws 20.0 milliamps, or about 100 times as muchcurrent. 0.22 milliamps is virtually nothing, but this is with the receiver sitting idle. What happenswhen someone pushes the bell?

Well, something interesting happens: When the button is pressed the receiver current jumpsto a little under 10 milliamps for about 200 to 300 milliseconds, then when it starts sounding thecurrent jumps to about 110 milliamps. The current fades back to it's idle level with the diminishing

tone of the chime. This is just a guess, but I think that probably the module that makes up thebrains of the receiver can power-down the 'made-a-sound-like-a-chime' part of itself, leaving onlythe RF section powered to conserve battery life. It would be interesting to know how much of thepower is used by the module, and how much is consumed by driving the speaker. Easy; we justremove the speaker from the circuit and measure the current again.

Repeating the measurement with one wire to the speaker unsoldered gives us the same 0.22milliamps idle current. When we push the button now, however, the current jumps to about 8milliamps and just stays there until it's done chiming. So, bottom line (and to use convenientlyrounded numbers), the receiver draws 10ma, and about 100ma is driven through the speaker.Now, if you're really paying attention, you will have noticed that this 100ma through the speakerdoesn't jive with the ohm's law figures above. Well, this is the difference between steady DCcalculations and pulsed or modulated DC. The figures above were a guess, and having made thecurrent readings we have real numbers so reworking the equations gives is an average of 0.300

watts, or 300 milliwatts going through the speaker. This is a MUCH more reasonable number. Allof this doesn't necessarily mean that we can just connect a 3-volt relay where the speaker is.Again, the signal going to the speaker isn't really DC, it's some sort of pulsed, or modulated DCshaped like the audio of a bell ringing. So we need to know a couple of things; what does thewaveform across the speaker look like, and is there enough constant DC in it to drive a relay. Weshould also know how much current can be driven through that transistor. A quick check of theC8050 data sheettells us that the transistor can handle up to 1.5 Amps so power handling isn't aproblem. Since we want to drive a relay, we'll have to find one rated to work at 3 volts. Running offof batteries and with a current capacity of better than an Amp through the transistor we reallydon't have to worry about the current draw of the relay. This could, however, be an issue running
http://www.datasheetarchive.com/search.php?q=C8050&sType=part&ExactDS=Startshttp://www.datasheetarchive.com/search.php?q=C8050&sType=part&ExactDS=Startshttp://www.datasheetarchive.com/search.php?q=C8050&sType=part&ExactDS=Starts


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it inside a PC.When the PC is on there is plenty of power available to run the receiver so turning the PC off

should be easy. But when the PC is turned off the only power available is the feeble amountavailable to let the power switch operate, and possibly to keep the ethernet circuitry operating (for"Wake On Lan"). Another participant in theMake Forum, "Lars-Phobe", offered that the currentAXT power supply spec calls for at least 10ma for the stand-by 5 volt power, but that 720ma isrecommended and that 1 to 2 amps is not uncommon. Hrmph. That's a hell of a range, anywherefrom 10ma to 2 amps! On the one hand I'm thinking that most manufacturers (I was in that field for15 years) take a recommended spec as the actual spec, so by Lars' number we have about a700ma budget. But some of that power needs to be used by the motherboard so if we can makethe receiver work at 200ma or less we should be fine. On the other hand the current spec is forcurrent computers, and we hackers / Makers have tons of old PCs lying around and I haven't aclue what the 1990 power supply spec was. Since I have a half dozen operating PCs here, andsome are pretty darned "vintage," I'll be doing some real-world tests.

My homework: Scope the points on the schematic and see what the signals look like. Read the specs on the power supplies in my PCs. And see if I have any 3V relays in my junk pile....errrr.... Piece Parts Inventory.

June 29, 2006 : Wave Riding

Well, I did find some relays buried in my inventory, I didn't read all the PC power supplies, andI did scope the receiver board. I decided that the power could wait. Let's just get a relay workingon battery power, and then we'll deal with PC power.

The first measurement, as marked "A" on the diagram above, is the output of the COB module tothe resistor which then drives the audio amp transistor. When the receiver first detects the signal,the output rises to about 0.60 volts. about 300ms later the chime starts. The chime signal is amodulated square wave that starts of oscillating between 0.60 volts and about 1.5 volts. As theringing fades, the amplitude of the signal drops until it is again floating at about 0.60 volts, andthen about 200ms later the output drops to zero.

I hadn't thought about it before hooking it up to the scope, but it actually cycles twice. Once

for the "ding", and once for the "dong". Since the ding is a higher frequency than the dong, thereis a bit of overlap in the middle of the complete cycle. The fact that during this overlap the outputis actually to different frequencies of square waves, my scope had a heck of a time syncing to thesignal.

The second waveform ("B") is from the other side of that resistor, and displays pretty much whatwe would expect. The signal is the same as before, but the resistor is limiting the current, and
http://forums.makezine.com/comments.php?DiscussionID=581&page=1http://forums.makezine.com/comments.php?DiscussionID=581&page=1http://forums.makezine.com/comments.php?DiscussionID=581&page=1http://forums.makezine.com/comments.php?DiscussionID=581&page=1


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therefore dropping the peak voltage to about 0.80 volts.Since this signal is applied directly to the base of the transistor, it controls how much current

flows THROUGH the transistor. That is, controls how much current flows from the +3Vdc of thebattery, through the speaker, and then through the transistor to the negative pole of the battery.(understanding that is going to become pretty important in a minute.

So, for example, if we lowered the value of the resistor, less voltage would be dropped acrossit, and our 0.80 peaks would move higher, say, to 1.20 volts. This would have the effect of makingmore current flow through the transistor, and thus more current flow through the speaker. Theend result? The sound from the speaker will be louder.

Next we look at the line between the transistor and the speaker ("C"). It's really the only one to

check because the other side of the speaker is the +3Vdc of the battery, and the other side of thetransistor is the negative pole of the battery.When the chime isn't chiming, the transistor is turned off, and there is no current flowing

through the speaker. That means that with the chime off we see the full +3 volts on both sides ofthe speaker. Thus, this waveform floats at the battery voltage. When the button is pressed, we cansee about a 0.20 volt drop on this line as the voltage applied to the base of the transistor rises to0.60 volts (not enough to turn it fully on).

Now consider things from the point of view of the speaker. When the chime is off there is+3Vdc on BOTH of it's leads so it "sees" 0 volts. When the chime first starts but has not startringing yet, the speaker still has +3Vdc on one side, but the other side has dropped to +2.80 VDCdue to that little voltage applied to the base of the transistor. With 3 volts on one side, and 2.8volts on the other side, the speaker "sees" 0.20 volts across its terminals.

Now, when the chime starts sounding the speaker has 3 volts on one terminal, and the otherterminal switches between 2.80 volts and about 1.80 volts. This means that the speaker "sees" asignal that oscillates between 0.20 volts and about 1.20 volts


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What this all boils down to is that at its maximum, there is only 1 volt across the speaker. That'splenty to make sound come out of a speaker. It is not enough, however, to operate a relay. Iguess that if you could find a relay out there with a 1 volt coil it would work. But in all my yearsI've never seen one.

Now I can hear a bunch of you yelling "What about a solid state relay???". Well, one of thebig selling points of solid state relays is their complete electrical isolation between the controlinput and the output, and that is done using an opto-isolator which is nothing more than an LEDaimed at a photo-transistor. 1 volt still isn't enough to drive the LED with any confidence.

What we need to do is to get that 1 volts expanded to as near 3 volts as we can.

And I already told you how to do it...where I was talking about how reducing the value of that resistor would increase the volume atthe speaker? Remember?

Exactly! As we reduce the value of the resistor the transistor "opens" more allowing morecurrent to flow, and thus more voltage is dropped across the speaker. So what value of resistor

do we want to put in there? Well, we're looking for the maximum current through the transistorso we want as high a signal to the transistors base as possible. Since any resistance in there isgoing to limit the current it's pretty obvious that to get the maximum current we won't replacethe resistor, we'll eliminate it completely.

Besides, it's a lot easier to short out the resistor that's on the board than it is to unsolderand remove it, and then solder a replacement onto the board.

Now this isn't completely without risk. We have no way of knowing how much current wecan pull out of that COB pin. I spent a LOT of time and there are simply -NO- datasheets or otherinformation available on the part. So, as it does with a lot of hacking, you come to a point whereyou just have to cross your fingers and dive it. Heck, the absolute WORST case is that I'll haveto go brave Home Depot again and buy another $14 doorbell.Up there ...

October 5, 2006 : I'm back

Well it's been a crazy few months. In short, I started a new business. Whatit is doesn't matter to you folks, but any of you who have gone through theordeal know what a huge task it is. Things are settling back down now soit's back to "work". :-)


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Earlier this week I got out the doorbell and transmitter. It's been sittingsince my last entry for this project and I was pleased to find that thebatteries are still good and the relay still works fine. To start to address thepower supply question, I removed the batteries and with test leadsconnected the battery contacts to a well regulated bench power supply.

As you might expect, the doorbell worked just fine with the power supplyset at 3 volts, the same as the batteries. I slowly increased the voltage, ahalf a volt at a time, testing at each voltage. I went as far as 6 volts and itstill worked. I didn't want to go any higher than that and chance blowingthe thing up. Besides, we're looking to run it at 5 volts inside a computer,and that is a VERY well regulated power supply. So, we're golden.

Then, just for giggles, I started turning the power supply down. Thereceiver worked all the way down to 1.5 volts, but relay stopped pulling inat about 2 volts. That's a good thing to know if you plan on using this modas a battery powered receiver. While I'm sure that in it's original state thedoorbell would actually sound down to 1.5 volts, our relay will only workdown to 2. That means that while you could expect the two "C" cells topower the doorbell for more than a year, with our relay mod the battery life

will be greatly shortened.

I have some down-time early next week, and I have an idle PC sitting inmy office, so I'll get this thing installed and then update the project.

If you want to be notified when this page gets updates, send an email to:dingdong(at)hackersbench(dot)com

If I were to rank the top evil things in the universe, the devil would besecond. Spam email would be first. You can absolutely be assured thatyour email address will be used for nothing else.

( By the way, third in the list is Microsoft) 8-)

where I was talking about how reducing the value of that resistor would increase the volume atthe speaker? Remember?

Exactly! As we reduce the value of the resistor the transistor "opens" more allowing morecurrent to flow, and thus more voltage is dropped across the speaker. So what value of resistordo we want to put in there? Well, we're looking for the maximum current through the transistorso we want as high a signal to the transistors base as possible. Since any resistance in there isgoing to limit the current it's pretty obvious that to get the maximum current we won't replacethe resistor, we'll eliminate it completely.

Besides, it's a lot easier to short out the resistor that's on the board than it is to unsolderand remove it, and then solder a replacement onto the board.

Now this isn't completely without risk. We have no way of knowing how much current wecan pull out of that COB pin. I spent a LOT of time and there are simply -NO- datasheets or other

information available on the part. So, as it does with a lot of hacking, you come to a point whereyou just have to cross your fingers and dive it. Heck, the absolute WORST case is that I'll haveto go brave Home Depot again and buy another $14 doorbell.


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BINGO!

As we said, with the resistor out of the circuit, the signal shown in the very first waveformabove is now applied directly to the base of the transistor, and the transistor is nowtoggling between fully open and fully closed. You'll notice that the signal doesn't actuallygo all the way to zero. This is because even when the transistor is fully turned on, it stilldrops a little voltage across it. In this case, about 0.18 volts.

So now our speaker "sees" 2.80 volts and then about 0.2 volts. This span gives us a

swing of 2.6 volts which is certainly enough to drive an LED, and is usually enough to pullin a 3 volt relay. But, we still have a problem.

It's not DC.

The power is being applied to the speaker (or our LED or relay) in pulses. In the speakerthis just makes sound. In an LED, the light actually turns on and off as fast as the signal,and this is going to cause some heartburn in a solid state relay. And for our conventionalrelay, you have to average the power across time and in this case the average doesn't addup to enough to actuate the relay.

Actually, when I tested this to get the waveform you could actually hear the chimesound coming from the relay as it struggled to close it's contacts. It was very quiet, but Icould hear it.

What we need is something to apply power to the relay during those gaps to bring the average


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up. Something that will save some of the power when it's there, and then give it back when it's

not. That is what a capacitor does.

If we place a capacitor between the speaker-transistor leg of the circuit and ground, it will (in

a manner of speaking) save up some power when the signal is there, and let it go when the

signal is gone.

(Okay, okay, it's not actually applying power, it's keeping that leg of the circuit from getting back

to +3 volts as quickly as it might. Some of you can congratulate yourself on picking a nit. The

rest, if you didn't understand this, don't worry about it)

Now I would like to be able to tell you that I applied my extreme mathematical prowess to

some complex formulas that I know by heart to calculate the value of the capacitor to insert into

the circuit. But that would be a lie, and life is too short to spend any of it in my attic looking for

my old high school electronics textbooks.

2.2 uF didn't work. 10uF didn't work. Neither did 22uF or 50uF.

Then I stuck a 100uF cap in the circuit ....

There ya' go. It's not the prettiest or cleanest waveform on Earth, but it will do. With the 100uFcapacitor in the circuit pressing the doorbell button resulted in a firm, authoritative, 'CLICK' ofthe relay pulling in. Well, actually, it was "CLICK" "CLICK". The relays closed twice for everypress of the button.

I have to confess that I had to stop and think for a minute before I figured it out....

"Ding" ... and then ... "Dong".

Oh jeeze! Brett wants this thing to turn on his PC, and I had no clue how computers respondedto double-clicks on their power buttons. What's more, this will make my flood lights into a kindof Disco strobe light. I started trying to think of ways to bridge the 300ms gap between clicksand then remembered the instruction sheet for the doorbell.

The doorbell will actually play different sounds for front and back doors (if you buy a second,optional transmitter). The sound for a back door is a single "Ding". The very last jumperposition on those headers in the transmitter tells it whether it's at a front door or a back door.I added a jumper to that position and the problem was solved. Pressing the button once


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clicked the relay once. Done!I've done all of the above testing using jumper leads and alligator clips. Let's clean that up

and make the mods to the receiver.

July 5, 2006 : Hackin'

Okay, the 4th of July weekend is over. One of the kids got bit by a dog that was scared bythe fireworks, plenty of sunburns to go around, I ate too much, drank too little, and though itwas a grand weekend I'm glad to be back at the bench instead of cleaning up the pool and the

backyard like I should.

To accomplish the relay mod we need to do three things ...

Short out the resistor at the audio amplifier (or replace it with a shunt). Add a 100uF capacitor to the circuit. Remove the speaker and replace it with a relay.

Finding the relay is probably going to be the biggest challenge. The one I have is from a lot Ibought on eBay about a year ago. I still see them from time to time, and a number of surpluselectronics on-line stores have something that will work. You're looking for a relay with a 3 VoltDC coil. Mine has a coil resistance of about 12 ohms, however anything in that ballpark shouldwork.

Since the receiver runs on 3 volts, just about ANY 100uF electrolytic capacitor will work. Youmight want to tend toward the lower voltages (i.e. 10v, 16v) because they are physically smaller.Up to this point the whole thing has been running with a bunch of alligator clip test leads

connecting the receiver, the batteries, the capacitor, and the relay. The receiver has been sittingpowered since I left it about a week ago and a quick check with a meter shows the batteries stillhave a full charge. I didn't expect anything else, but it's nice to confirm that my changes to thecircuitry didn't start killing the batteries.

I'm just going to list the steps of the hack here as opposed to writing them all out. You canfollow along with the pictures.


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Remove the receiver board, unsoldering the wires to the battery terminals. This just makes iteasier to work with the board. Solder a wire shorting out resistor R11. This is easiest if you solder a straight piece of wire toone end, then use a fine tipped needle-nosed pliers to bend it into shape, and then solder the

other end. Now solder one lead of the capacitor to the end of the same resistor closest to the middle ofthe board. I found a short piece of scrap wire and stripped about 2 inches of it. Not for the wire,but the insulation. You can slide the insulation over the leads of the capacitor before soldering itinto place. Solder the other lead of the capacitor to a ground point on the circuit board. I found that Icould slide lead into the same hole and along side the black wire that goes to the batteryterminal. Now we need to assemble the relay. I found that originally there was not enough room for therelay inside the case of the receiver, but then figured out that if I removed the clear case of therelay it fit just fine. Also, I like my projects to have a finished appearance so I dig out a 1/8"mono earphone jack I had salvaged out of something ages ago. This jack will carry the relaycontacts to the outside. Solder the earphone wires to the relay contacts, and two pieces of wire

to the coil leads of the relay. Unsolder and remove the two wires on the receiver board that used to go to the speaker, andreplace them with the wires that come from the coil of the relay.

Drill a hole in the side of the case to accept the earphone jack. Install the jack, and hot-gluethe relay into place in the bottom of the receiver case.

Reconnect the power wires from the receiver board to the battery terminals. I replaced thewires in mine with longer ones just to make my life a little easier.

Reinstall the receiver board in the case, snap the cover on, and your done!

Just a few random notes...

At a few points through assembling this thing I stopped and reconnected everything with thetest leads just to make sure it still worked. This may seem silly to some, but doing so will help

you avoid "finish line failure". That is, getting everything assembled, putting in the batteries,snapping on the cover, and having it not work. Then you have to backtrack and figure out atwhich step you made your mistake.

I thought about adding a power switch, but since the receiver will run for about a year on apair of "C" cell batteries I saw no real need. There is room, however, under the circuit boardalongside the earphone jack to accommodate a micro-toggle switch if you want one.

You could also add another jack for the antenna if you like. A little panel-mount BNC, "F", orbanana jack should also fit under the circuit board. If I have trouble with range I'll go ahead andadd that later. But for now, 50 or 60 feet of range should do fine.

It might not be a bad idea to add a diode in parallel with the relay to shunt any back-current

from the field of the relay coil collapsing when power is removed from it. This would protect thetransistor from a big reverse voltage spike. However, I didn't see anything nasty on my scopewithout it, and I left it out to save current for the relay.

Another addition would be an LED indicator that lights when the relay closes. I would beconcerned that the LED would draw too much of the available current and the relay would nolonger close. If you want it, give it a try. I have to leave *SOMETHING* for you folks to figure out.8-) Besides, the closed receiver case makes a really good resonator, and you can definitely hearthe relay clicking.


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I wired the earphone jack to the Normally Open (NO) contacts of the relay, so pressing thebutton will turn something on. You could, alternatively, wire up the Normally Closed (NC)contacts so that pushing the button would turn something off. Better still, instead of a monoearphone jack you could use a stereo headphone jack, bringing both the NO and NC terminalsout of the receiver making the device even more universal. What would you want to turn off

remotely? Well, the stereo in our kid's car when it get's within 100 feet of our house is the firstthing that comes to mind. (grin)

Next up ... we'll get this thing running off of a PC power supply.

July 5, 2006 : Hackin'

Okay, the 4th of July weekend is over. One of the kids got bit by a dog that was scared bythe fireworks, plenty of sunburns to go around, I ate too much, drank too little, and though itwas a grand weekend I'm glad to be back at the bench instead of cleaning up the pool and thebackyard like I should.

To accomplish the relay mod we need to do three things ...

Short out the resistor at the audio amplifier (or replace it with a shunt). Add a 100uF capacitor to the circuit. Remove the speaker and replace it with a relay.

Finding the relay is probably going to be the biggest challenge. The one I have is from a lot Ibought on eBay about a year ago. I still see them from time to time, and a number of surpluselectronics on-line stores have something that will work. You're looking for a relay with a 3 VoltDC coil. Mine has a coil resistance of about 12 ohms, however anything in that ballpark shouldwork.

Since the receiver runs on 3 volts, just about ANY 100uF electrolytic capacitor will work. Youmight want to tend toward the lower voltages (i.e. 10v, 16v) because they are physically smaller.

Up to this point the whole thing has been running with a bunch of alligator clip test leadsconnecting the receiver, the batteries, the capacitor, and the relay. The receiver has been sittingpowered since I left it about a week ago and a quick check with a meter shows the batteries stillhave a full charge. I didn't expect anything else, but it's nice to confirm that my changes to thecircuitry didn't start killing the batteries.

I'm just going to list the steps of the hack here as opposed to writing them all out. You canfollow along with the pictures.

Remove the receiver board, unsoldering the wires to the battery terminals. This just makes iteasier to work with the board. Solder a wire shorting out resistor R11. This is easiest if you solder a straight piece of wire to

one end, then use a fine tipped needle-nosed pliers to bend it into shape, and then solder theother end. Now solder one lead of the capacitor to the end of the same resistor closest to the middle ofthe board. I found a short piece of scrap wire and stripped about 2 inches of it. Not for the wire,but the insulation. You can slide the insulation over the leads of the capacitor before soldering itinto place. Solder the other lead of the capacitor to a ground point on the circuit board. I found that Icould slide lead into the same hole and along side the black wire that goes to the batteryterminal.


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Now we need to assemble the relay. I found that originally there was not enough room for therelay inside the case of the receiver, but then figured out that if I removed the clear case of therelay it fit just fine. Also, I like my projects to have a finished appearance so I dig out a 1/8"mono earphone jack I had salvaged out of something ages ago. This jack will carry the relaycontacts to the outside. Solder the earphone wires to the relay contacts, and two pieces of wireto the coil leads of the relay.

Unsolder and remove the two wires on the receiver board that used to go to the speaker, andreplace them with the wires that come from the coil of the relay. Drill a hole in the side of the case to accept the earphone jack. Install the jack, and hot-gluethe relay into place in the bottom of the receiver case. Reconnect the power wires from the receiver board to the battery terminals. I replaced thewires in mine with longer ones just to make my life a little easier. Reinstall the receiver board in the case, snap the cover on, and your done!

Just a few random notes...

At a few points through assembling this thing I stopped and reconnected everything with thetest leads just to make sure it still worked. This may seem silly to some, but doing so will helpyou avoid "finish line failure". That is, getting everything assembled, putting in the batteries,

snapping on the cover, and having it not work. Then you have to backtrack and figure out atwhich step you made your mistake.

I thought about adding a power switch, but since the receiver will run for about a year on apair of "C" cell batteries I saw no real need. There is room, however, under the circuit boardalongside the earphone jack to accommodate a micro-toggle switch if you want one.

You could also add another jack for the antenna if you like. A little panel-mount BNC, "F", orbanana jack should also fit under the circuit board. If I have trouble with range I'll go ahead andadd that later. But for now, 50 or 60 feet of range should do fine.

It might not be a bad idea to add a diode in parallel with the relay to shunt any back-currentfrom the field of the relay coil collapsing when power is removed from it. This would protect the

transistor from a big reverse voltage spike. However, I didn't see anything nasty on my scopewithout it, and I left it out to save current for the relay.

Another addition would be an LED indicator that lights when the relay closes. I would beconcerned that the LED would draw too much of the available current and the relay would nolonger close. If you want it, give it a try. I have to leave *SOMETHING* for you folks to figure out.8-) Besides, the closed receiver case makes a really good resonator, and you can definitely hearthe relay clicking.

I wired the earphone jack to the Normally Open (NO) contacts of the relay, so pressing thebutton will turn something on. You could, alternatively, wire up the Normally Closed (NC)contacts so that pushing the button would turn something off. Better still, instead of a monoearphone jack you could use a stereo headphone jack, bringing both the NO and NC terminalsout of the receiver making the device even more universal. What would you want to turn offremotely? Well, the stereo in our kid's car when it get's within 100 feet of our house is the firstthing that comes to mind. (grin)

Next up ... we'll get this thing running off of a PC power supply.

October 5, 2006 : I'm back

Well it's been a crazy few months. In short, I started a new business. What it is doesn't matter to you


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folks, but any of you who have gone through the ordeal know what a huge task it is. Things are settlingback down now so it's back to "work". :-)

Earlier this week I got out the doorbell and transmitter. It's been sitting since my last entry for thisproject and I was pleased to find that the batteries are still good and the relay still works fine. To start toaddress the power supply question, I removed the batteries and with test leads connected the battery

contacts to a well regulated bench power supply.

As you might expect, the doorbell worked just fine with the power supply set at 3 volts, the same as thebatteries. I slowly increased the voltage, a half a volt at a time, testing at each voltage. I went as far as6 volts and it still worked. I didn't want to go any higher than that and chance blowing the thing up.Besides, we're looking to run it at 5 volts inside a computer, and that is a VERY well regulated powersupply. So, we're golden.

Then, just for giggles, I started turning the power supply down. The receiver worked all the way down to1.5 volts, but relay stopped pulling in at about 2 volts. That's a good thing to know if you plan on usingthis mod as a battery powered receiver. While I'm sure that in it's original state the doorbell wouldactually sound down to 1.5 volts, our relay will only work down to 2. That means that while you couldexpect the two "C" cells to power the doorbell for more than a year, with our relay mod the battery life

will be greatly shortened.

I have some down-time early next week, and I have an idle PC sitting in my office, so I'll get this thinginstalled and then update the project.

If you want to be notified when this page gets updates, send an email to:dingdong(at)hackersbench(dot)com

If I were to rank the top evil things in the universe, the devil would be second. Spam email would befirst. You can absolutely be assured that your email address will be used for nothing else.

( By the way, third in the list is Microsoft) 8-)

October 6, 2006 : Email Feedback

Dave B. wrote ...

Nice hack, and a very clear instructable!

One thing you mentioned was a kick-back suppression diode. A lot of relays come with the diode built-

in, but if you want to add one, place it across the relay coil, not across the 100 uF cap, as you

suggested. Wire it with the cathode (+) terminal at the +3V, the anode at the collector of the 2SC8050.

Dave

Oops. I must have been typing faster than I was thinking that day. I've changed the 'capacitor' to 'relay'

in the above text. Thanks Dave! Both for pointing out the typo, and the compliments.John


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Heath Doorbell MOD

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Transcript of Heath Doorbell MOD