I'd like to control a slave clock, via a relay(s), which needs an alternating polarity pulse once a second.

The attached circuit incorporates two SPDT relays which do this (one relay activated one second, the other the next, to provide the alternating polarity).

I'd like to use a SSR relay though to avoid mechanical wear.

I've got some SPST DC-DC SSR relays but, as they're SPST, I can't see a way to replicate the circuit I've attached.

Does anyone have any ideas?


*Note: I can control the SPDT relays from a Raspberry Pi using a transistor circuit (output 5V). The SSR relays can be controlled using the output from the Pi without using a transistor (output 3 - 5V). The voltage the clock requires is around 24V DC.*

  • \$\begingroup\$ If the load only needs 6V, there are plenty of other solutions... How much current does this "clock" need? \$\endgroup\$
    – bobflux
    Mar 31, 2017 at 19:11

2 Answers 2


I would recommend an H-bridge driver.

If you've not familiar with them, they typically are implemented in a single IC or an IC with external power FETs, depending on the current you want. I don't know your load current but I'd guess it'll suit the former, a single IC.

Have a look at the ONSemi NCV7729 (5..28 V, 8 A) or ST L9958 (4..28 V, 8.6 A) to give you some ideas.


What you are looking for is an H-Bridge circuit.

You can use four SPST-SS Relays as shown below.


simulate this circuit – Schematic created using CircuitLab

DO NOT press both buttons at once though. A centre off, momentary on left and right, toggle switch would be more appropriate.

However there are more "integrated" solutions for this.

  • \$\begingroup\$ Made me smile that you suggested relays after yesterday's solenoid exchange, with genuine warmth though. Hey, you don't have to shout "DO NOT" in capitals but you could spell 'once' properly. In little letters :-) :-) \$\endgroup\$
    – TonyM
    Mar 31, 2017 at 22:19
  • 1
    \$\begingroup\$ @TonyM. I try to tailor my answers to the information in the question and not suggest alternatives that may be beyond the OPs skill level. OP stated he had a bunch of SPSTs so I answered it as is. Thanks for pointing out the other thing. Respect back at you. :) \$\endgroup\$
    – Trevor_G
    Apr 1, 2017 at 14:32
  • \$\begingroup\$ Thank you Trevor - this is what I was looking for. I got the circuit working fine at first but now something odd's happened after I tidied up the wiring (all connections are fine though). Everything's fine when RLY1 and 4 are on, but when RLY2 and 3 are on, there's a short circuit on the 9V load. Do you have any ideas why? Thanks! \$\endgroup\$
    – David
    Apr 3, 2017 at 18:42
  • \$\begingroup\$ @David They should operate 1&4 or 2&3. 1&3 should never be on at the same time. Also check your relays are all "Normally Open" type. \$\endgroup\$
    – Trevor_G
    Apr 3, 2017 at 18:46
  • \$\begingroup\$ Sorry, I meant 2&3. I've just checked and the relays are the correct type. \$\endgroup\$
    – David
    Apr 3, 2017 at 18:52

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