Considering the below PCB with the following scheme, with components inserted, with DC+ connected to a relay's DC+, DC- to DC-.

When CH1 is connected to the relay, the voltage between HV1 and HGND is ~2.8V.

When CH1 is not connected to the relay, the voltage is, as it should be, ~4.8V.

The relay is a solid state high level trigger.

The bidirectional logic level translator is of this kind.

I tried this with different logic converters, and different relays, but the behavior is the same, what could be the problem? I'm struggling with this for some hours and I can't figure it out.

enter image description here enter image description here

  • 2
    \$\begingroup\$ G3MB has only 440 Ω input impedance. The logic level converters are designed to work with logic chips. Most likely they won't work with those SSRs. And since you only need one unidirectional signal your best option is to use simple BJT or MOSFET switch. But first try connecting 5V to CH1, to make sure your relay works (disconnect it from level translator first, of course) \$\endgroup\$
    – Maple
    Sep 10, 2018 at 23:30
  • 1
    \$\begingroup\$ @Maple: I'm glad you were willing to decipher those diagrams, but please do not answer the question in comments, as this bypasses the normal review process for answers, as discussed in meta \$\endgroup\$
    – Dave Tweed
    Sep 11, 2018 at 12:19
  • \$\begingroup\$ I agree, it's a very good answer that helped me a lot, if @Maple has some time to detail in an answer would be even better \$\endgroup\$ Sep 11, 2018 at 14:33
  • \$\begingroup\$ @DaveTweed I did not think it would qualify as an answer, more like musings on the problem \$\endgroup\$
    – Maple
    Sep 11, 2018 at 17:07

1 Answer 1


Those bidirectional translators are designed to work with logic chips. Below is a typical schematics (taken from Sparkfun). As you can see, they work similar to open drain circuits, i.e. the outputs are pulled high by relatively large resistors, and only actively driven low.

enter image description here

Your G3MB relay has only 440 Ω input impedance and they require at least 10 mA LED current to switch on. 10k pull-up on level translator can source only about 0.5 mA.

So, get rid of that 4-line bidirectional translator. You only need one signal and in one direction only. Use simple high-side switch to control SSR.

N-Channel M1 should have logic level gate voltage (something under 2.5V), P-channel M2 should be rated at least for 30 mA.


simulate this circuit – Schematic created using CircuitLab

Note that this only works if your LGND and HGND are the same common GND. The translator that you use has them connected internally. You did not specify how you convert 5V to 3V. If it is LDO or Buck then your grounds should be connected already, if it is isolated converter you need to connect them manually.

There are many alternative solutions, for example optocoupler here. You might even be able to reuse that level translator of yours if you connect its output to the gate of M2 in the schematics above. In this case the translator will replace both resistors and M1. The control signals on GPIO2 should be reversed though, with 0 being "on".


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