I want to connect a Chamberlain 942FP / 379LM fingerprint reader garage door "keypad" to a Chamberlain B750T (I think a.k.a. B750) opener. Unfortunately the fingerprint reader is much older than the opener, so I couldn't pair it with the opener wirelessly.
Fortunately however, the fingerprint reader comes with a receiver which plugs into an AC outlet and can be connected with 2 wires to the opener. This receiver also has a push button to allow for easy testing without having to know if the fingerprint aspect is working, and when pushing it, I could hear a relay click, but unfortunately it doesn't appear to be compatible with the opener as nothing happens.
I hooked up a multimeter to the wires I connected to the receiver unit and can see a voltage change when pressing the button (appears to be AC). The instructions for the receiver unit say it doesn't matter what wire is hooked up to where on the opener (i.e. red vs white), but the push buttons that came with the opener unit have terminals specifically labeled white and red, and the inputs to the opener have color coded wire hookups. Also the push buttons that came with it do not plug into AC power, so I'm assuming they draw power from the opener itself through the low voltage bell wire.
I suspect the opener needs some DC signal whereas the receiver that goes with the fingerprint reader gives out an AC signal. Any ideas on how to make them compatible (buying something is fine if needed).