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Advice please: controlling a transistorised relay board with anopen collector output


Frank Sweeting

Jan 1, 1970
I'm not up with modern electronics & I'm struggling a bit with this: if
anyone can give me any advice or suggestions I'd be grateful

I have a device with an open collector output which can be controlled ON/OFF
remotely by phone. I'm told I have to be careful about the amount of current
supplied by the open collector output so I chose to use the open collector
output to switch a relay with a transistorized relay board to trigger its

I've got a 1.5k resistor between 12V on the device & its open collector
output, the output then goes to the trigger on a relay board. When the
output is ON theres 10.5V on the trigger, & when its OFF theres 0.7V on the

Powering up the device has the open collector output ON & the relay is
triggered ON, BUT, when I command the Open collector OFF the trigger
voltage only drops to 0.7V, & the relay stays ON
So I haven't got control of the relay as I wanted because I cant turn it

I've experimented & found I have to get the trigger voltage down to about
0.1V before the relay goes OFF

So heres the question, how can I get the open collector output down to 0.1V
or if I can't is there a work around

TIA Frank
Frank Sweeting

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