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I'm using a 555 IC in a timing circuit. If I tie the trigger low and power up the unit can I get a single 4-5 second pulse from the output? Will the unit reset when power is removed? Is anything else needed in the circuit?

D Trimble
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1 Answers1

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You wouldn't ground the trigger, you would have a capacitor from the trigger to ground and a resistor from trigger to the positive supply. When powered on the capacitor would start out discharged which would pull trigger low starting the timing cycle, the capacitor would then charge through the resistor making trigger high. This will give you a single pulse each time power is applied. The pulse time would be determined by a capacitor and resistor on the threshold and discharge pins, you tie those pins together and connect a capacitor from them to ground and a resistor to the positive supply.

See this answer which shows the resistor and capacitor for power up triggering as well as a circuit to allow it to be manually re-triggered. If you only need it to trigger on power up you can leave out the components to the left of R1 and C1.

This circuit should give you a high output for around 5 seconds and then the output will go low until power is cycled. The you just have to interface it to your relay. I think you can drive a PMOS from the 555 output and use that to switch the relay.

schematic

simulate this circuit – Schematic created using CircuitLab

GodJihyo
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  • Thank you for your input. However, I need to explain a couple of things. I'm producing a simple on delay timer. When the 555 is powered on it starts timing. after the time period it goes low. I'm putting an inverter circuit on the output of the 555 so it starts low and after the timing period goes high this will drive the gate of a mosfet transistor which powers a relay. The relay stays on till the power to the 555 is removed. Then the 555 resets. – D Trimble Apr 30 '23 at 23:23
  • @DTrimble See the edit I made to my answer. You can add any additional information you have to your question and that might get it re-opened. – GodJihyo May 01 '23 at 16:41
  • I ran into a snag with the output of the 555. I need to source 12VDC to a relay and LED instead if sinking my load to ground. Can I use a P channel MOSFET connected from the output of the 555 to it's gate to turn it on when the 555 goes low? Can I source voltage instead of sinking it? – D Trimble May 06 '23 at 00:43
  • I found some F9540N MOSFETs. Can I use them in the circuit? – D Trimble May 06 '23 at 17:23
  • I also have some BD136 PNP medium power transistors. – D Trimble May 06 '23 at 18:21
  • A PNP or PMOS should work. – GodJihyo May 06 '23 at 20:16