How to Modify a Radio Shack Pager for Use as a Backup Ejection System

To use the Radio Shack pager as a backup ejection system, you must modify the pager to fire a flashbulb. The circuitry in the pager can not directly do this, so a circuit must be added to the pager that can.

When the pager receives a page, it flashes a number in its display and sounds a beeper (actually a piezo-electric transducer). The signal activating this beeper is a very short pulse train that re-occurs at the beep interval. The external circuit that is added takes these pulses, amplifies them, and uses the amplified signal to fire an SCR (silicon controlled rectifier). A SCR is basically a switch that will turn on when a small signal is presented on its gate. Once turned on, it will not turn off again until current through it is interrupted. Therefore, once turned on, it stays turned on until your ejection charge fires. When the charge fires, the circuit is opened, and the SCR turns off.

The first step is to open the pager case, remove the piezo device, and route its connection to the outside of the case for attachment to the external circuit. After testing the pager, remove the battery cover and the two screws under the batteries. The rear cover lifts straight up and off, but there are two snap-in tabs still holding the case together, one on each side near the top. Place the pager face down and press in on the sides of the rear cover near the top, or use a small screwdriver to gently pry out on the sides of the front cover near the top. Be aware that the two battery contacts on the left side are attached to the circuit board and will remain with the front part of the case and printed circuit board. The two contacts on the right will stay with the rear cover.

Now with the rear cover off, locate the piezo beeper. It's the approximately 1/2 in. diameter round thing in the top right corner of the board. You'll need to unsolder this and connect two wires in its place. To do this remove the PC board from the front half of the case. It isn't held by anything but the switch and the 7 segment LED at the top that displays the paged number. Be careful when teasing the board out, the switch and LED connections to the main board are flexible, but can be damaged easily. Also pay attention to how the slide switch is installed, it's the hardest part to put back in the case.

With the board out of the case, unsolder the piezo beeper and solder two wires in its place. You can use a pair of twisted wires, or a small shielded cable. The negative lead is the bottom connection, farthest away from the top of the pager. Now re-assemble the pager, being careful to get the switch and the LED display back in their proper place. You'll have to drill a hole in the back cover to bring out the wires you have soldered in place of the beeper. Test the pager once again, keeping the two new wires you have installed separated. Now on to the external circuit!

The assembly of the external SCR board is not critical. The most important thing is to keep the wires from the pager to the SCR board as short as possible, and place components Rx, Cx, and Q1 as close to each other as possible. After assembly of the SCR board, check your wiring carefully before connecting the battery.

To test the completed pager system, connect a flashbulb ONLY to the pager. Turn on the pager, then switch the Safe/Arm switch to Arm. Send a page, and you should see the flashbulb fire in about two seconds.

Ideally, the SCR board would be mounted to the pager case with standoffs. This will give you a one piece, compact package. The Arm/Safe switch should be whatever type you are comfortable with; I have used toggle switches (with the toggle cut off very short) and slide switches in similar applications. In any case, the Arm position should be toward the aft end of your rocket. Good Luck!

Direct any questions to: Jack Goff (cgoff@scsn.net) TRA#4260

External SCR Board Schematic

This makes me nervous. The circuit counts on the leakage current of Q1 being less than the current required to fire the SCR. If Q1's leakage increases, the SCR could fire prematurely. Transistor leakage increases with an increase in temperature (as does the gain/sensitivity of the SCR) and I can just picture the rocket waiting out on the pad on a warm day when all of a sudden the ejection charge goes off....

A 1K resistor from Q1's emitter to ground would go a long way to improve the stability of this circuit, as would a small (~.01 uF) cap between the SCR's gate and cathode.