Twilight Zone - Trough Proximity Sensor ModificationsNOTICE: For some very important information, please read the Technical Article Notice and Disclaimer, found on our Technical Articles Index page before performing any of the work described below. The information given below assumes you have read the Notice and Disclaimer first.
This modification adds a small PC-board mount pot, or trimmer, to the sensor PC board, which allows you to always be able to adjust the sensitivity of the sensor.
Step 1: You'll first need to obtain a small PC-mount trimmer pot. We recommend the lowest resistance rating you can get (something around 100 ohms would be ideal, but the most common "low rated" pots are about 1k (1000) ohms). Some of these small pots can be very, very touchy, so it's best to get one that has a low resistance rating, and allows you a good accurate adjustment. The average amount of resistance you'll want from the pot is around 20 to 30 ohms, so check your pot with your meter first to make sure it will let you adjust it easily to this value. See image below to see the pot that we used.
Step 2: Remove the sensor from the game. Two hex-head screws hold it to the underside of the playfield. Make sure power is turned off while doing all modifications!
Step 3: On the component side of the board, cut the trace between the connector pin and the sensor. This is the only trace on this side of the board, so you can't miss it. Use a sharp knife, or X-Acto blade, to slice through the trace. Use multimeter to make sure there is no continuity after you've made the cut- if there is, cut some more.
Step 4: Scrape some of the insulation off the trace that leads to the sensor (see image above). Remove enough to adequately solder a jumper wire onto the bare metal of the trace.
Step 5: On the solder side of the board, use a small marker to mark the position of the 3 legs of the trimmer pot onto the PC board, and drill three holes in the board through which you will mount the pot. Use a very small drill bit.
Step 6: Install legs of pot through the holes you drilled in the PC board, then bend the legs on the other side of the board to hold the pot on the board.
Step 7: Back on the component side of the board, connect two small jumper wires to the pot. One jumper wire will come from the back side of the pin on the connector (the one with the trace going away from it), and the other jumper wire will come from the other side of the trace that you cut, where the insulation was scraped away. Connect the other ends of the jumper wires to the pot- one goes to the middle leg, and the other goes to either side leg (doesn't matter which side leg).
Modification complete. At this point, you may want to make the third modification as described below, before re-installing the sensor board into the game.
Re-install the sensor board and the cable that goes between it and the driver board. With the sensor board installed, the pot should be easily accessible with a small flat blade screwdriver (see image at top). Power up the game, install one ball in the ball trough (up against the sensor), and adjust the pot until the LED on the driver board just turns ON. This indicates that the sensor has detected the metal ball. Move the ball back into the trough and the LED should go OFF on the driver board. Adjust the pot accordingly.
Note: Either Modification #1 or Modification #2 should be made- not both. Read through each mod first to determine which you would prefer to make.
This modification adds a resistor to the sensor PC board, which makes the sensor more sensitive.
Step 1: You'll first have to determine what size resistor to add to your sensor board. This can vary from game to game, and depending on how sensitive, or insensitive your sensor currently is. The best way to determine the size of resistor to add, is to splice a potentiometer into one of the sensor wires that go between the driver and sensor boards. Adjust the pot until the sensor reliably detects a metal ball in the trough, then remove the pot you spliced in, and take a resistance reading across it. Most games that we have done this on usually require a resistor in the range of 20 to 30 ohms, 1/4 watt. Your mileage may vary...
Follow steps 2, 3, and 4, as described above.
Step 5: On the solder side of the board, connect one end of the resistor to the end pin on the connector which does not have a trace coming from it. Connect the other end of the resistor to the leg of the sensor that is not connected to the trace on this side of the board.
Modification complete. At this point, you may want to make the third modification as described below, before re-installing the sensor board into the game.
Re-install the sensor board and the cable that goes between it and the driver board. Power up the game, install one ball in the ball trough (up against the sensor), and the LED on the driver board should turn ON. This indicates that the sensor has detected the metal ball. Move the ball back into the trough and the LED should go OFF on the driver board.
Technical Articles
Twilight Zone - Trough Proximity Sensor Modifications
