See photo #1, when plenty of light shines on the photocell, resistance is 12 Ω, the relay does not respond. In photo #2 the photocell is removed and the relay responds as expected. I can add a transistor and make this circuit work but my question is, why this doesn't work as indicated in photo "1?

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Obviously the coil in the relay has some effect.

Information edited in from comments:

  • The relay is an SRD-05VDC-SL-C from Songle Relay.
  • Its coil voltage is 5 V.
  • The photocell is about 12 Ω when fully illuminated.
  • The supply voltage is 5 V.
  • The circuit works fine if the photocell is replaced by a 15 Ω resistor.
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    \$\begingroup\$ What is the relay's rated coil voltage and resistance or required current. Also what is the photocell's resistance in full light -what resistance do you consider as "virtually nil"? What voltage are you applying to the circuit? \$\endgroup\$ Jul 1 at 15:37
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    \$\begingroup\$ Please add extra info to the question and not in the comments. Also trying uploading image #1 again. \$\endgroup\$ Jul 1 at 16:02
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    \$\begingroup\$ We still need to know what "virtually nil" means in actual numbers. \$\endgroup\$
    – brhans
    Jul 1 at 16:35
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    \$\begingroup\$ A part number for the relay would be nice. \$\endgroup\$
    – Hearth
    Jul 1 at 16:40
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    \$\begingroup\$ @RomanKierzek I've edited all this information into the question, but in the future, please do that yourself. Does the datasheet I found look correct to you? Are you sure it's not an SRD-05VDC-FL-C? The -SL-C is sealed, and yours doesn't look sealed. \$\endgroup\$
    – Hearth
    Jul 1 at 17:30

3 Answers 3


You haven't provided any information about the CdS photocell. \$12\Omega\$ is quite small. Since \$15\Omega\$ ohms will pull the relay in, then the photocell's resistance is higher than \$15\Omega\$.

There are two reason's for this:

  1. There is not enough light.
  2. The minimum illuminated resistance of the photocell is too high.

Be careful. The relay turn off indictive kick can ruin the photocell. Use a diode across the relay coil

  • \$\begingroup\$ Are photocells that sensitive? I thought they were effectively just blocks of bulk semiconductor, and would be about the same sensitivity as your average thermistor; i.e. pretty robust. \$\endgroup\$
    – Hearth
    Jul 1 at 22:04
  • \$\begingroup\$ @Hearth: The absolute max voltage for CdS cells ranges from approx. 100V to approx. 400 V in my experience. So they are robust, but still worth a calculation to determine the height of te splke. \$\endgroup\$
    – RussellH
    Jul 1 at 22:24
  • \$\begingroup\$ @RussellH Yes, resistance too high, don't have specs for the photocells I have so likely all have high resistance. Will need to find an appropriate cell. Thank you for your input. \$\endgroup\$ Jul 2 at 0:39

Virtually nil and truly nil are two very different things.

NOTE: Without the relay coil current, or power, or resistance data, this is all speculation.

If, for example, the required relay coil current is 200 mA, then the coil resistance is 25 ohms. The CdS cell forms a voltage divider with the coil resistance. If the "on" resistance actually is 12 ohms (a very low value), then only 3.8 V appears across the relay coil. The resulting coil current is only 135 mA, and probably is not enough current for proper operation.

If you edit your post with a link to the relay datasheet, that would help. Sometimes the minimum coil voltage needed for guaranteed operation is given there.

  • \$\begingroup\$ Tried all 5 of my photocells (have no specs) and only 1 volt, more or less, downstream of photocells. Thank you for your input. Will look for a proper cell. \$\endgroup\$ Jul 2 at 0:32
  • \$\begingroup\$ If you measure the resistance of the relay coil (out of the circuit), you can calculate the cell resistance that yields 1 V across the coil in-circuit. Using the datasheet numbers, I get 280 ohms. That is a much more reasonable value for that size cell. \$\endgroup\$
    – AnalogKid
    Jul 2 at 15:07

The part number of the relay indicates that it is model SRD, 5VDC, sealed, high sensitivity, and one set of form C contacts. For the high sensitivity version coil resistance is 70\$\Omega\$ and pull-in voltage is 75% max, so figure 3.75 V.

Add the 12\$\Omega\$ and coil current will be 61 mA and voltage 4.27 V, so technically it looks like it should work. That it doesn’t could indicate that either the relay specs aren’t correct, it’s a different relay (yours doesn’t look sealed) or the photocell resistance is higher than stated. Based on the relay pulling in with a 15\$\Omega\$ resistor I'd say it's the latter.


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