# Can I use electrolytic capacitor in a schmidt trigger oscillator

I want to build this cricket chirping circuit that I found on Happy Lab.

The values are specified as follows

R1: 100 k
R2: 39 k
R3: 290 k
C1: 100 nf
C2: 10 µf
C3: 10 µf
C4: 100 µF
D1,D2: 1N4148
IC: 74HC14 (6x inverter with Schmitt trigger)


I have sourced all the parts except am having trouble getting 10uF ceramic capacitors for C2 and C3 in time for my deadline. Happy Lab application notes say

Since the use of polarized capacitors for C2 and C3 is kind of unskilled (it works most the times especially when using higher voltage capacitors ie. 63 volts) it is better to use non-polarized capacitors. 10µF non-polarized ones are expensive and or bigger. So much smaller capacitors can be used when increasing the value of the corresponding resistor at the same time.

I don't follow this. Does this mean I can use 16V 10uF electrolytics (which I do have) for C2 and C3? I am aware electrolytics can explode with reverse polarity which obviously would be bad.

If I can use the electrolytics, which side of C1 is +ve? The trigger output or diode facing side? My analogue circuitry is very rusty.

I plan to drive this circuit not with a solar panel but 2 x AA's in series (3V).

• No it says it's not a good idea. It also says you can use 1/10 the capacitance (1 uF) if you increase the resistors 10x (R2=390k, R3=2.9M) R3 is an odd value, should it have been 390k or 270k? – Brian Drummond Aug 27 '15 at 11:33
• yes i thought R3 290k strange but I don't have the skill to understand the design myself. Was going to use a 270k+20k in series – fiat Aug 27 '15 at 12:50

I would just lower the capacitor values C2 and C3 by a factor 10 and compensate by multiplying the values of R2 and R3 by a factor 10. The 74HC14 is CMOS so it will also work with such an increased impedance.

If you keep the R x C value constant then timing will not change (within reason but the values I propose are still OK).

Then the largest timing capacitor becomes 1 uF which is easy to find in a non-polarized version.

For C4 you can just use a polarized electrolitic capacitor.

You can use two 22uF capacitors in series (back to back). Or bulky and expensive film capacitors. Or unpolarized electrolytics,

Or use 100V 10uF aluminum electrolytics and hope for the best. Polarity does not matter since it reverses, higher voltage caps can tolerate some reverse voltage. They won't explode if insufficiently rated, your circuit just won't work.

But really, now, this is 2015 and nice X5R 10V 10uF 0805 capacitors cost something like 15 cents in small quantity.

• thank you. the cost of the caps isn't the issue, just the timing of obtaining them in time. will try and source the correct ones then. – fiat Aug 27 '15 at 12:48