I took apart an old magnetic door alarm, where when the contact is broken it counts down from 10 seconds for the user to input the code (which ranges from 1 to 8, there is no 0 or 9 key). This alarm is so old there is no information on the internet. I took it apart, and so far all of the internal components have been working, which is great for a hobbyist such as myself!
I am currently trying to reverse engineer it with just a 9v battery and a multi-meter, but I am having some trouble.
On the front it appears to say 894HB MEYER, and DG4 - 102 - D.
Four of those pins used to be connected to an IC labelled LS 7220 LSI 8122 PM. This was used to check the correct pass-code was entered in the correct order. The other pins were not connected.
In the first picture, at the bottom, you see the eight pins. According to the original plastic alarm the pins are connected to buttons in this random order: 5 1 6 2 3 7 4 8
I connected the white wire to the negative terminal of a 9v battery. I did a continuity test where I connected one side of my multi-meter to the other 9v battery terminal, and then touched each one of the eight pins. Strangely they all had continuity, even when I did not push any buttons.
Could someone tell me how it works so I can use it for my DIY projects?