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I am working on a latching push-on/hold-off circuit for a momentary button and I am stuck with determine which type of capacitors to use. The circuit is detailed below (originally from Mosaic Industries):

schematic

simulate this circuit – Schematic created using CircuitLab

I am unsure whether or not this will work with electrolytic capacitors, and if it will which polarity should they be? I have considered ceramic capacitors, but I am unable to easily source 1uF ceramic capacitors. As the schematic does not indicate that they are polarised capacitors I suspect that ceramics are the only option, but if this is the case what value would be ideal given I can only find values less than 1uF.

For this circuit I am using a CD4011 NAND IC.

Any advice is greatly appreciated.

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  • \$\begingroup\$ For both an electrolytic capacitor will work just fine. For C1 there is no way that the voltage on the R2 side will be lower than the voltage on the R1 side. An issue might be leakage current of C2 since R3 has a high value. Just use a new (not a used one) capacitor for C2 and it should work fine. \$\endgroup\$ – Bimpelrekkie Jun 28 '16 at 7:17
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    \$\begingroup\$ Oh and there's a fundamental error in the circuit as you have drawn it. The output of a 4011 NAND is by far not strong enough to make a 100 ohm lightbulb light up. A NAND like that can provide about 10 mA so a LED is possible but not much more. For a lightbulb you will need to add a transistor, I would use an NMOS for that. \$\endgroup\$ – Bimpelrekkie Jun 28 '16 at 7:19
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The big capacitor and resistor at the bottom of the circuit are used to produce a 5 second RC time constant so that to get the lamp to turn off one has to press and hold the switch for a long time (~ 3 seconds). The smaller RC components at the top of the circuit act so that a quick button press will turn the lamp on again.

You don't say what value ceramic capacitors you do have, but if you cut the capacitor values in half you will cut the time constants in half. If you want to restore the time constants simply double the resistor sizes.

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