In learning electronics, I tried to create an LED blink circuit using a 5V relay (56 Ω coil resistance at 5V), a 5V power source, a 50 ohm resistor connecting NC to the relay electromagnet, COM to power, and an LED with 200 ohm resistor on NO as in the pic. Everything works and the relay rapidly opens and closes the circuits and the LED goes on/off rapidly. Great!

But if I then add a 1000 uF polarized capacitor across the relay (in the right manner with the stripe on the '-' side), everything stops - no buzzing sound - the LED stops lighting up and I see no voltage drop across its legs. Once I remove it the relay buzzes repeatedly again… The capacitor works, I tested it with a little LED and resistor on a separate bread board…

  • I measure a 2V difference on the multimeter on the capacitor's legs (does not change while I keep the power on)
  • I measure 0 millamps (using multimeter in line with the circuit) The breadboard connections are tight...Where else may I have gone wrong? (I am pretty sure I am doing the measurement right)
  • I measure 29 MegaOhms on the capacitors legs (that seems high?) The breadboard connections are tight...Where else may I have gone wrong?

BTW - the resistor in the pic is 100 Ohms - sorry - I am using a 50 Ohm one though.

Pic of BBoard

EDIT: Once I took in all the comments/answers, if I swap out the 50 Ohm resistor for a 10 Ohm resistor and I swap out the 1000 uF capacitor for a 470 uF Capacitor (I already swapped it for another 1000 uF with the same effect - maybe it's a low quality lot...I don't know), I get the 'slower/human visible' blink effect I was after.

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    \$\begingroup\$ Fritzing will allow you to generate a schematic rather than the breadboard cartoon. You will have to untangle what it gives you into a nice drawing. You might as well learn how to do it as the schematic is the language of electronics. \$\endgroup\$
    – Transistor
    Commented Dec 11, 2023 at 23:50
  • \$\begingroup\$ A 50 ohm resistor is too much to reach the required 3.3 V at the coil. \$\endgroup\$
    – Jens
    Commented Dec 12, 2023 at 0:23
  • \$\begingroup\$ Note that this circuit you've made is pretty hard on the relay; it won't survive doing this for more than a few hours, days at the most. \$\endgroup\$
    – Hearth
    Commented Dec 12, 2023 at 0:28

1 Answer 1


Either the capacitor is leaky, in other words low resistance and is not allowing the voltage across the coil to get high enough or the relay does not respond well to the slowly changing voltage caused by the capacitor. Change the resistor in series with the coil to a lower value, 33 ohms perhaps.


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