Digital Meets Analog: 555 Timer Chip Case Study · Case Study 555 Timer Chip 2017-10-30 PHYS351001...

Digital Meets Analog: 555 Timer Chip Case Study

2017-10-30 PHYS351001 L10 Michael Burns

555 Transistor level

Vcc = power supply

Case Study 555 Timer Chip


There’s actually a chip with all that stuff in it.

Vcc = power supply



Let’s see what we can do using things we already know.

+ -

+ -

+ -

Vcc

Output

Comparators Flip-Flop

Voltage follower

Transistor (acting as “on-off switch”)

Voltage Divider

Vcc = power supply



How does this work?

+ -

+ -

+ -

Vcc

Output

Vcc = power supply



How does this work?

Vcc

2

3Vcc

1

3Vcc

Vcc = power supply



How does this work?

+ -

+ -

Vcc

Lower comparator’s output depends on whether

voltage at 2 > 1

3Vcc or <

1

3Vcc.

2

2

3Vcc

1

3Vcc

Upper comparator’s output depends on whether

voltage at 6 > 2

3Vcc or <

2

3Vcc. 6

8

1



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

Lower comparator’s output depends on

whether Vc1 > 1

3Vcc or <

1

3Vcc.

Upper comparator’s output depends on

whether VC1 > 2

3Vcc

or < 2

3Vcc.

So flip-flop Q output toggles depending on the comparators.

So flip-flop Q output toggles opposite Q. Transistor turn on (low-Z between C & E, grounding 7) or off ( high-Z between C & E) depending on Q .

8

1



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

8

1



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

8

1

If this transistor is “off” (high-Z between C & E).

Then C1 charges via this path. (From 1

3Vcc to

2

3Vcc)



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

8

1

If this transistor is “on” (low-Z between C & E).

Then C1 discharges via this path. (From 2

3Vcc to

1

3Vcc)



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

8

1

If this transistor is “on” (low-Z between C & E).

Then C1 discharges via this path. (From 2

3Vcc to

1

3Vcc)

Need R2> ½ R1 or can’t discharge

from 2

3Vcc to

1

3Vcc



How does this work?

+ -

+ -

Vcc

2

1

3Vcc

6

7

3 Output

2

3Vcc

8

1



How does this work?

+ -

+ -

+ -

Vcc

2 (trigger)

6 (threshold)

7 (discharge)

3 (output)

8 (+Vcc)

1 (ground)

5 (control voltage)

This capacitor reduces noise on the voltage divider.



There’s actually a chip with all that stuff in it.

Chip has one extra pin from out circuit, which allows one to reset the flip-flop.



555 Die



Can also get a dual version (LM556), with two 555’s inside.




Literally is two 555’s with common Vcc & ground.)



556 Die



Specs • Timing of seconds through hours per period. • Astable and monstable modes (one shot or oscillatory) • Adjustable Duty cycle (i.e. “one” time & “of” time don’t need to be the same • Output can source or sink up to 200mA • Output & supply TTL compatible ( but can also run with Vcc up to 18 volts) • Temperature stability better than 0.005% per degree C. Applications • Precision Timing • Pulse Generation • Sequential Timing • Time delay genertion • Pulse Width Modulation • Pulse Position Modulation • Linear Ramp Generation



Monostable (one shot)

Vc

Output

Trigger Input

Every time the trigger is pulsed low, one cycle completes.

1.1RAC



Astable (oscillates) We’ve tied the trigger (2) to the timing capacitor to get it to self trigger.

Vc

Output

Time to charge, t1 = ln(2) (RA+RB)C Time to discharge, t2 = ln(2) RBC Total Period, T = ln(2)(RA+2RB)

Duty Cycle = RB

RA+2RB

(fraction of period “high”)



Pulse Width Modulator (PWM) We’ve taken the monostable and removed the control voltage capacitor.

This pin goes to the 2/3 node of the voltage divider.

Output

Trigger input

Modulation input

Every time there’s a trigger pulse, the output is a pulse whose width is proportional to the modulation voltage.



Pulse Position Modulator We’ve taken the PWM and tied the trigger to C.

Output

Pin 5 input

Density of output pulses is proportional to the modulation voltage.



Intrinsic 50% square wave generator

Same as astable, but use the output to charge and discharge the capacitor through the same resistor.

R

Period T 1.4 RC

Digital Meets Analog: 555 Timer Chip Case Study · Case Study 555 Timer Chip 2017-10-30 PHYS351001...

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