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I need help with the design of a timer delay circuit for the output of a sensor circuit which can keep the output HIGH for an adjustable time range (about 1 to 10 min), after the digital output of the sensor circuit goes LOW. Also, the timer should be restarted automatically if the output of the sensor circuit goes HIGH again in the middle of the timer operation.

I saw many different power off delay timer circuits which used the 555 IC, for example, this circuit, but I can't find a resettable timer circuit.

More details: in my case the sensor circuit is a temperature circuit with a LM35 sensor and after it triggered on adjusted temperature, the output went HIGH (12V) and if the temperature suddenly drops below the trigger point, the output will go LOW as well(it contains hysteresis). I need a time delay after the output of the temperature circuit in order to keep the output HIGH for the adjusted time range, and, if the output of the sensor circuit goes HIGH again, the timer interval's must be restarted and it must wait until the signal goes LOW again(like in the motion sensors's delay circuit which id active while it's sensing motion)

-I know it is easy to do it with a microcontroller but I still need to learn more programming .
-It's not possible for me to learn programming at the moment ,so maybe a retriggerable monostable is what I want ?

Below is diagram of the temp. sensor:

schematic

simulate this circuit – Schematic created using CircuitLab

schematic according to RoyC answer:

schematic

simulate this circuit

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  • \$\begingroup\$ Well it's either learning how to design 555-based circuitry, which frankly is a dead end these days, or learning microcontrollers. Given those options, I actually think you should not do this with 555 but get some cheap microcontroller board (maybe some arduino clone) and get started with those. \$\endgroup\$
    – Marcus Müller
    Commented Jan 7, 2017 at 10:11
  • \$\begingroup\$ @Marcus Müller I disagree, there is a tendency these days to reach for the microcontroller as soon as anything gets in the least complicated a 555 circuit will do what you very simply. Learning about microcontrollers will give you the ability to solve a large number of problems but learning the 555 will give you the ability to solve a certain class of problems very simply. \$\endgroup\$
    – RoyC
    Commented Jan 7, 2017 at 11:04
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    \$\begingroup\$ @RoyC I agree that understanding of how and why the 555 works and a bit of analog craftmanship is really worthy of learning! However, if someone with no electronic design (not: rebuilding circuits someone else designed) experience which skill to acquire first, I'd say it's dealing with microcontrollers, simply it gives one more versatility for this kind of tasks. Of course, being an analog engineer is probably a talent rarer than being someone who knows how to program µCs. \$\endgroup\$
    – Marcus Müller
    Commented Jan 7, 2017 at 12:02

1 Answer 1

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To use a 7555 here you have to deal with two problems, first you have a high going control signal and second how do you retrigger the 555.

The first is fairly easy you use a simple transistor inverter to invert the input signal.

The second takes a little more understanding of how the 555 is working. It charges up the timing capacitor until it reaches a threshold voltage and then turns off. You can retrigger it by discharging the capacitor.

Look at this circuit.

enter image description here

Q1 and R1 invert the incoming signal to make trigger active low Q2 discharges the capacitor while the input is high giving you the retrigger function. Note with this circuit your timing period is from the input signal going low.

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  • \$\begingroup\$ I'm wondering that when the signal coming from sensor is HIGH, and Q1 and Q2 is Active, so timing Capacitor C1 is discharged, Also the threshold pin is connected to Ground through Q2, and Trigger pin is also active as connected to Ground through Q1. in this situation the trigger pin try to activate 555 and threshold try to deactivate it?! what will happen? which will win? \$\endgroup\$
    – rocknroll
    Commented Jan 8, 2017 at 11:13
  • \$\begingroup\$ Ok, the 555 returns to the off or low output state when the threshold is high not low. As we are holding it low there should be no problem. \$\endgroup\$
    – RoyC
    Commented Jan 8, 2017 at 11:22
  • \$\begingroup\$ You're right, 555 activate when threshold is high. one another question, how you choose R1 and R2 values (10k)? \$\endgroup\$
    – rocknroll
    Commented Jan 8, 2017 at 12:14
  • \$\begingroup\$ That is actually a really good question. To tell the truth I did not think about it much it was based on experience. Now let me try to justify it. R2 has to load the output of your open collector sensor. If it is very large the fet gate and stray capacitance starts to slow down the response. If it is very small it overloads the output of the sensor leading to either failure of the sensor or a bad voltage waveform on the gate. A small value here also results in a larger current from the supply. 10k is a good middle value but is just an approximation. Similar arguments apply to R1. \$\endgroup\$
    – RoyC
    Commented Jan 8, 2017 at 12:33
  • \$\begingroup\$ What if our trigger pulse lasts 5 seconds and we want the output not interrupted by second trigger (continuous high output) and just an extension in timer? \$\endgroup\$
    – AKTanara
    Commented Aug 11, 2023 at 17:41

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