DIY Capacitor TesterBy Lance Summers
Magnifying Glass Alternative to Reading Capacitor LabelsAfter trying to go through and make sense of all the various caps in the Jameco Capacitor Grab Bag, I decided that I needed a better way than just a magnifying glass. So I put together a Cap Tester that uses a 555 timer chip to make a specified chirping sound based on the cap value under test. After putting the schematic together on a breadboard, I built a PBC version.
1 PCB Circuit Board (min 24 x 18 hole)
1 LM555C IC Chip
1 SPST Momentary Push Button Switch
1 0.01µF Capacitor 
1 100µF Electrolytic Capacitor
1 9V Battery Clip
1 6 or 8-pin Female Header
1 6 position DIP Switch
2 Red LEDs
1 NPN Transistor [2N3904]
1 30mm Round PCB Speaker
1 330Ω Resistor [Note: Lot of 100]
1 100Ω Resistor [Note: Lot of 100]
2 1KΩ Resistors
2 10KΩ Resistors
1 100KΩ Resistor
1 1MΩ Resistor
1 10MΩ Resistor
Hookup Wire (22 or 24AWG)
The tester is designed to chirp at a rate corresponding to the capacitor, the smaller the cap, the faster the chirp (fast enough that it actually blending into more of a beep). The larger caps will make very distinctive chirps.
The DIP switch allows the circuit to step up or down by a power of 10. The smaller the resistance, the larger the cap that can be tested. For example, a 1µF cap will not make a sound and the signal LED will stay on if the 1Ω or 100KΩ resistors are alternately used but if you use the 100Ω or 1KΩ, then you can hear the chirp.
To use my tester, select the resistance via the DIP switch that will give the range of sound that you expect for the value of the resistor.
In my Jameco grab bag, most of the caps were in the 100µF to 1000µF range, so the 1MΩ resistor or the 100KΩ resistors were switching into circuit to give the distinctive sound.
This method was very handy for the caps that had labels such as 330. It's hard to tell if it's really a 331 or was it a 33Ω. The sound generated from these caps proved them to be 33, whereas the 520 caps were really 521 and made a slightly lower sound with the 100K switched into circuit.
After some trial and error, I made the following modifications to the schematic:
1: I used an 8-pin, female header with seven of the eight pins going to ground and the eighth one going to pin 2 of the 555 timer. This allowed the cap to be easily plugged in without worrying about making the leads fit.
2: I used an 8-position DIP switch to switch in the following resistors in place of the 4.7MΩ resistor: 10M, 1M, 100K, 10K, 1K, 100Ω. This gave me a wider range of caps to test.
3: I changed the 470KΩ to a 10KΩ resistor to shorten the discharge time.
4: I also added a power LED so that I know when the circuit is on.
I keep the following caps with the tester so that I can use them as sound references: 10pF, 100pF, 1000pF, .01µF, .1µF.
Get familiar with the sound of each of these at their switch level and the one next to it. Once you are familiar with the sound of each, then when you test a cap you will immediately know the range it fits in because the sound will be higher or lower than the referenced cap. Lower sound indicates the cap value is larger than the referenced one. If the sound is higher, the cap value is lower than the referenced cap. With this method, a whole grab-bag can be separated into bins of ranges in very short order.
How to read a capacitor
If you have questions, email Lance Summers at [email protected].
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