Don't mind the diy battery tray ;PMy son and I took apart a small electronic toy and were wanting to see the inside. Once we cracked it open I saw this circuit and couldn't figure out what each piece did. I think I understand the purpose of C2 (reducing noise from the motor but I'm unsure about C1 and C3. Thanks!

Note: This is my first time ever making a schematic. It is likely inaccurate.

Secondarily, when I measure voltage near the capacitors (from battery to switch) the readings seem odd. My measurements are in the low mV range and fluctuate. Why is that? EDIT: I updated the schematic based on feedback. The thought is that C1 is a PTC and my schematic was inaccurate.


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Welcome to EE.SE! Your schematic is almost 100% certain not correct around C1. Please check again. \$\endgroup\$
    – winny
    Commented Oct 21, 2019 at 14:54
  • \$\begingroup\$ Thanks @winny any tips as to why? I am guessing at the 5pF based on the 050 marking on the side but this is certainly how it is wired. \$\endgroup\$
    – ktilcu
    Commented Oct 21, 2019 at 14:55
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    \$\begingroup\$ Both the value and the connection is doubtful to make the circuit useful. Do you have a photo? \$\endgroup\$
    – winny
    Commented Oct 21, 2019 at 14:56
  • 2
    \$\begingroup\$ Based on the schematic, C1 is probably a polyfuse. \$\endgroup\$ Commented Oct 21, 2019 at 16:24
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    \$\begingroup\$ I vote for PTC too. If you edit the schematic and rotate C2 and the motor 90 degrees and draw both plus on the top and minus/ground as straight lines, it will become much more clear. C3 is for EMI/noise suppression. \$\endgroup\$
    – winny
    Commented Oct 21, 2019 at 17:12

1 Answer 1


Capacitor C1 seems strange, also I don't see any resistor for the LEDs which makes me think the C1 is actually a resistor (or it should be).

Explanation: See comments of ktilcu/KingDuken below: It looks like a PTC, (in this case) stands for "positive thermal coefficient" which typically is an algorithm used for measuring heat. The higher the heat, the more resistance you have. "C1" could actually be a thermistor that could create a current limit.

Capacitor C3 is a so called bypass capacitor going from VCC to GND and is typically 100 nF. It keeps the voltage more stable in the circuit and is typically used for ICs. Normally it's placed as close as possible to the VCC (+) side.

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    \$\begingroup\$ An algorithm for measuring heat? I would have said that a PTC thermistor is a resistor whose value depends on its temperature. As the temperature goes up, so does the resistance. Since current flowing in the device generates heat (i.e., increases its temperature), a PTC thermistor is useful as a crude current limiting device. \$\endgroup\$ Commented Oct 21, 2019 at 16:00

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