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I'm trying to make a circuit that will press the reset button on the device after a power failure.

This should reset the device when the power is restored and remains inactive until the next power failure.

This is my first idea but with this scheme the device is reset every 2 minutes. Reset circuit

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The circuit operates by charging the capacitor C1 trough the base of the transistor. As long as the voltage over the capacitor is not too large, your transistor will conduct. When this voltage is larger than 24 minus 0.7 volts, the transistor will stop to conduct. Ideally it would stay this way. However, trough paracitic resistance in your capacitor, it will slowly discharge and finally (aparrently after 2 minuts) the transistor will turn on again.

You can however do the following:

You can make a circuit that turns on a transistor for some seconds when it is powered up. You can best connect the 24V supply to the same power source as the device you want to reset. When the circuit is turned on, your device will automatically reset.

For example, you could do it this way:

schematic

simulate this circuit – Schematic created using CircuitLab

Here R1 and C1 make the time delay until transistor 1 is turned on. When transistor 1 is turned on, transistor 2 will turn off and your relay stops actuating the reset button.

If you don't want to use the second transistor you could even place your relay at the location of R2, getting this circuit:

schematic

simulate this circuit

But in that case you would need a relay with NC contacts and the relay gets powered all the time (which isn't very power efficient of course).

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  • \$\begingroup\$ Unfortunately there's a constant 24v (battery power). The device requires reset after loss of main power supply (220V AC). \$\endgroup\$ – Milenkovic Jan 26 '15 at 20:52
  • \$\begingroup\$ You have (at least) two options then: Connect the optocoupler in series with R1 (or even replace R1 with the optocoupler) or use an adapter or small power supply connected to the 220V AC to replace the 24V supply. I would suggest the second option, because it won't need batteries. \$\endgroup\$ – Douwe66 Jan 26 '15 at 20:55
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The main source of your problem is that the opto-isolator is turning Off and On at every zero crossing of the incoming AC signal.

Couple of questions:

1) How long can the reset time of the circuit be? Note that this is not the duration of the signal generated by the circuit but rather, how long does incoming power have to go away in order for the circuit to reset itself?

2) What duration of output pulse do you require? I assume that because you want a reset signal for something, that signal should be a pulse rather than a maintained signal.

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  • \$\begingroup\$ It takes just one second without main power to the device reports an error and it is necessary to reset it . For the output pulse is enough any time between 5 and 15 seconds. It can be a constant signal since it activates a relay that takes the role of reset button. \$\endgroup\$ – Milenkovic Jan 26 '15 at 21:22

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