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Sunday, February 17, 2013

Retriggerable 555 timing circuit: An interesting find

A friend of ours is working on a commercial project for low cost timing module. He request us for some help regarding designing a Mono-Shot timer. In general the circuit was using a NE555 in mono shot configuration. The problem was that he needed the mono-shot to start at power up and also it should be re-trigger-able with a button. And all this with the minimum set of components.

We took up this challenge to help him out. The first job was to analyze the existing circuit. So we took his permission to publish a small part of his circuit, he gladly agreed since it was very generic part any way.
Initial Circuit: 555 Mono-shot Timer
Lets describe the components(Starting from Left Side):
  • SW1 - Switch to Trigger the Mono-shot
  • R1-C4 - For deciding the ON - Timing  t = 1.1(R1 x C1)
  • R2 - Pull-up Resistor for the Trigger Pin
  • C3 - Start-up Capacitor for the Trigger pin
    What you might say here is why is R2 = 2.2MEG OHM ??
    This is one trick. We know that at start-up the capacitor C3 is discharged and hence acts as a Short to ground. This means that at start-up the Trigger pin is low which triggers 555 at start-up.The abnormally high value of R2 ensures that it takes some time to charge the C3 to 2/3 VCC. The time period for trigger to be above the 2/3 VCC is tr = R2 x C3 x ln(3/2) = 89.2mS
    This ensure that the 555 is triggered after power ON.
  • C1,C2 are decoupling capacitors.
Now in the above circuit we are able to have trigger the mono shot at the  start-up and also trigger it using a switch. However we are still not able to RE-Trigger the timer. This was solved by a simple modification.
Improved Re-trigger-able circuit
Now this circuit uses an additional circuit Q1. The purpose of adding this transistor is to forcefully discharge the timing capacitor C4 in order to re-start the timing.
This circuit worked fine initially. However our friend still pressed for making something similar. Additionally we observed that there is a slight anomaly that is ignored in many cases from the above circuit. When the voltage on the C4 is rising in the ON Period if the button SW1 is pressed the voltage at C4 must fall to 0v. However for Q1 to turn ON the voltage between Emitter of Q1 to its Base must exceed -0.7V(PNP). This might not occur in some cases. In that case the timing from the button press to the re-triggering gets reduced and is not achieved till the voltage difference reaches above 0.7V. The we again had to analyze the circuit and workup the formulas. The next circuit was the best we could come up with.
Final version of Re-Trigger-able 555 with minimum components
This circuit solved the problem. It completely restarted the timer to extend the timing. Here the Reset pin as well as Trigger pin are channeled by the same R2-C3 combination. This this was the lowest parts count 555 Re-trigger-able circuit. Our friend was very happy with the performance of this solutions and is presently incorporating it into his design.

This was one of the design challenges that we solved. We wish more such people such as our friend, who are facing design level issues; can collaborate with us. These exercises although not for profit help us to learn and understand the concepts deeper.
Let us know your thoughts and suggestions.


  1. Although this is an old thread I thought I should point out something... By connecting the trigger switch to pin 4 RESET, would that not reset the internal flip flop and bring the output pin 3 low, which is my interpretation of the 555 functionality? I have a one-shot timer circuit that needs to be retriggerable but I have not yet tested this configuration.

    1. Yes you are right, by connecting the trigger switch to pin 4 RESET you will have pin 3 OUTPUT go LOW when the TRIGGER is grounded (i.e. when the switch is closed). When you release the switch, the timer will restart. So it will not be a smooth continuous output, it will always go LOW when triggered and go HIGH again when the trigger is released. That doesn't suit my application, but it may suit others.