The liftmaster 3800 and 3900 jackshaft openers are basically the same just with different options shipped standard in the packaging. Both are now obsolete after only being on the market for a few years giving way to new openers with whiz bang features, internet connectivity, etc.
My 3800 started experienced the "phantom opening" problem where the door randomly opens. This has been seen with the newer openers as well. There have been product replacements (but no recalls) and the problem lies in the wall control panel. The 3900 ships without a control panel, instead just has a single non lighted pushbutton. Thus I replaced my failing control panel on my 3800 with a non lighted doorbell button. No more worries about phantom opening when a replacement control panel fails.
Being curious about possibility of using a lighted doorbell, I hooked the Oscilloscope up to the wires to take a look see what's going on.
Warning: technical detail to follow...
The signal with the button is 35V and when the button is pressed it goes to 0V and draws 4.6mA as can be seen in the "doorbell pressed" waveform. I connected up the control panel and observed the communication. What I saw was that the panel normally draws about 2.5mA and the communication method to the opener is to short these power wires to ground.
The conclusion is that if you install a lighted doorbell that draws 4.6mA or more the opener will think the button is continually pressed. Likely, the lighted doorbell would have to draw 2.5mA or less.
So if your 3800 or 3900 is having the phantom opening problem, just replace your control panel with a non lighted pushbutton. The 3800 / 3900 has the vehicle remote receiver inside the opener itself. Some newer models have the receiver in the control panel, so using a doorbell button isn't an option.
Brian
My 3800 started experienced the "phantom opening" problem where the door randomly opens. This has been seen with the newer openers as well. There have been product replacements (but no recalls) and the problem lies in the wall control panel. The 3900 ships without a control panel, instead just has a single non lighted pushbutton. Thus I replaced my failing control panel on my 3800 with a non lighted doorbell button. No more worries about phantom opening when a replacement control panel fails.
Being curious about possibility of using a lighted doorbell, I hooked the Oscilloscope up to the wires to take a look see what's going on.
Warning: technical detail to follow...
The signal with the button is 35V and when the button is pressed it goes to 0V and draws 4.6mA as can be seen in the "doorbell pressed" waveform. I connected up the control panel and observed the communication. What I saw was that the panel normally draws about 2.5mA and the communication method to the opener is to short these power wires to ground.
The conclusion is that if you install a lighted doorbell that draws 4.6mA or more the opener will think the button is continually pressed. Likely, the lighted doorbell would have to draw 2.5mA or less.
So if your 3800 or 3900 is having the phantom opening problem, just replace your control panel with a non lighted pushbutton. The 3800 / 3900 has the vehicle remote receiver inside the opener itself. Some newer models have the receiver in the control panel, so using a doorbell button isn't an option.
Brian
