I want to create a circuit with a SPST toggle switch that when flipped in one position the output signal is high and when flipped in the other the output has a momentary low pulse and then rises back to a high output again. It seems very simple but I cannot for the life of me figure it out.

  • 1
    \$\begingroup\$ Connect the switch to a monostable multivibrator (555?) \$\endgroup\$
    – Eugene Sh.
    May 25, 2016 at 17:58
  • \$\begingroup\$ SPST has opened/closed position so which one you choosen? \$\endgroup\$ May 25, 2016 at 18:24
  • \$\begingroup\$ There is of course wide awareness of the 555, but it's also worth looking at other possibilities like the 74xx121 or`221 \$\endgroup\$ May 25, 2016 at 22:16

3 Answers 3



simulate this circuit – Schematic created using CircuitLab

Figure 1. Crude pulse-low on switch closure.

You could play with this in a simulator or try it out. You haven't given any clues what the load is but if it's in the mA range this might work.

  • R1 supplies current to Vout. Vout is normally high.
  • R2 discharges C1. Both sides are at 5 V.
  • SW1 pulls the bottom of C1 low. The top side will be pulled with it causing Vout to drop close to 0 V.
  • If SW1 is held closed C1 will charge via R1 with time constant \$\tau = R_1C_1 \$. This will determine the width of your pulse.
  • When SW1 is released R2 will discharge C1 again and it will be ready in 3 to 5\$\tau\$ or about \$ 5 R_2C_1\$.

Solution 1

A simple 1 transistor circuit:

enter image description here

When SW1 is closed it pulls one side of C1 to 0V. The other side of C1 is at 0V so in order to maintain the voltage across C1 it pulls this plate to -V.

D1 (a general purpose diode) prevents this negative going edge exceeding the base-emitter breakdown voltage of T1 for supply voltages greater than about 5V.

This plate of C1 then charges back up through R2 to about 0V.

When the switch is closed that side of C1 rises immediately to +V causing the other side of C1 to do the same (can't instantly change the voltage across the capacitor). This side of then C1 discharges through D1 and Vbe in parallel with R2 causing T1 to switch on for a short time.

Once this plate voltage falls below 1.2V (2 x 0.6V drops) T1 turns off (again) leaving the output voltage at the collector of T1 high.

Solution 2

The 555 monostable

As it stands the standard monostable needs to be tweeked before it will answer this question.

enter image description here

Tweek 1 - is to produce a short negative edge pulse from the switch. (R1,C1,D1 and R2).

R2 holds the trigger and threshold inputs high. When the switch is closed the negative edge of the pulse is transferred across C1 pulling the voltage at C1, R2 low - triggering the 555.

Opening the switch will not trigger the 555 and any positive edge pulse produced is clamped by D1 to no more than 0.6V above the supply.

R3 and C2 control the timing of the pulse (standard timing elements- adjust values as required).

C3 is optional but recommended (smoothing of internal divider resistor chain)

Tweek 2 - The output of the 555 is normally LOW with the pulse going high. As you require the opposite (high-low-high) a simple transistor inverter circuit is added.

  • 1
    \$\begingroup\$ In that case the 74xx121/221 ends up being a simpler circuit, as it has outputs in both polarities. \$\endgroup\$ May 25, 2016 at 22:09
  • \$\begingroup\$ @ChrisStratton I wasn't planning on answering this question but this answer (using a 555) was put up in response to the number of comments about using a 555, all of which seemed to fail to point out that a 'standard monostable circuit' (as suggested) was unsuitable. A 74xx121/122 does produce a simpler solution but you are then limited to a 5V (TTL) supply. \$\endgroup\$ May 26, 2016 at 13:11

Use a 555 timer.

There are example circuits in the link below for monostable multivibrator. This means that it triggers once (mono). There are other modes you can get, such as continuous square wave, which is astable multivibrator. In this case the toggle switch could be replaced with a SPST if that's all you have. I won't go through the details because the link has a very good explanation for everything.


  • \$\begingroup\$ links die, better to put up an actual answer \$\endgroup\$ May 25, 2016 at 19:19
  • \$\begingroup\$ There has gotta be hundreds of explanations for 555 timer setups; there is no reason to redo it. \$\endgroup\$
    – jbord39
    Jul 13, 2016 at 1:47
  • \$\begingroup\$ Very true but (a) Answers here should be stand alone not just net references and (b) the standard 555 monostable output gives a positive pulse output and the questions requires a negative pulse. The reason I made the comment was simply to help you put up an answer that wouldn't get down voted ( btw It wasn't a downvote from me as I rarely downvote). Over time, as folks come across your answer it is liable to attract more down votes and that is a very good reason to redo it. \$\endgroup\$ Jul 13, 2016 at 12:55

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