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I'm using a 7555 timer in monostable configuration to control a load of about 1A.

If I disconnect the 1A load or use a bench supply it works fine, but the drop in voltage from the batteries seems to be preventing the timer from resetting after the expected duration. The discharge pin voltage levels out at the control pin voltage but the timer doesn't reset.

I'm using 3xD batteries which provide about 4.5 V open circuit and 3.7 V with the 1A load.

Not sure what make of timer so can't confirm its minimum working voltage.Timer circuit

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    \$\begingroup\$ can you upload thecomplete image and also the datasheet of the timer? \$\endgroup\$
    – User323693
    Aug 31, 2019 at 18:03
  • \$\begingroup\$ also, what transistor specifically are you using? \$\endgroup\$ Aug 31, 2019 at 18:59
  • \$\begingroup\$ Your 555 appears to be missing a GND connection (pin-1). \$\endgroup\$
    – brhans
    Aug 31, 2019 at 19:10

2 Answers 2

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As it turns out, the timer was operating correctly, and the fault was with the rest of the circuit:

Circuit as it was not working

I was designing the circuit to trigger the timer in response to a sudden increase in light levels. The op-amps use voltage references derived from the battery voltage. When the MOSFET turned off, the rise in battery voltage meant the timer was immediately triggered again. The trigger was only 20µs which is why I missed it.

Maybe I should leave the question up as an example of why to post the entire schematic.

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  • \$\begingroup\$ Please mark the answer as accepted, because it solves your issue. \$\endgroup\$ Feb 7 at 7:08
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The 555 is triggered by a logic-low pulse on its trigger input. It will not time out unless the trigger pin voltage goes back to being at logic high.

Here is how adding a capacitor and a resistor will do it:

[![555 timeout][1]][1]

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    \$\begingroup\$ Please provide a citation or link for the content that you copied into your answer. We want to be sure to give credit to the original creator. \$\endgroup\$ Aug 31, 2019 at 20:45

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