Fig. I: The opened radio, showing the speaker wires.
Fig. J: Radio leads: blue to call button signal, orange to
speaker signal, orange/white to ground.
Find the contacts for the radio’s call button (not
the push-to-talk switch, but the button that sends the
radio’s page tone). Solder leads to the ungrounded
side of this button, the speaker signal contact, and
ground (I took it from the LCD housing). Secure all
contacts with glue (Figure J).
Attach the 3 leads to a 1" stereo jack, connecting speaker to tip, call to ring, and ground to sleeve
(Figure K). Reassemble the radio. If you can’t fit the
jack inside the radio, glue it to the outside.
6. Assemble the time-delay
Following the schematic (Figure M), mock up the
arrangement of the components in the project
enclosure, and make holes for the switches and wires.
You’ll need to trim the project board and the inside
of the enclosure to make everything fit (Figure L).
The control circuit is built around a BASIC Stamp
microcontroller, which has 8 input/output pins
(Figure N). The mini project board holds the parts
in place, and wires are used to solder them together.
Mount the microcontroller in the SIP socket so that
it won’t be damaged by the heat of soldering (first
trim away the SIP’s 2 extra holes).
Using the wires from the radio’s stereo plug,
154 Make: Volume 15
Fig. K: Speaker signal, call signal, and ground leads
connect out to the 1" stereo jack. Fig. L: The board
needs trimming to fit into the controller box.
connect the radio’s ground to the microcontroller’s
ground pin (VSS) and to the negative side of the 9V
battery, then connect the microcontroller’s VIN lead
to the positive side of the battery. Connect the VSS,
PCO, and PIC pins to the 3-pin header so that you can
download the control program to the BASIC Stamp.
Connect P0 (on lead 7) to VDD (+5V) via the 10kΩ
resistor, then connect it to the 2N2222 transistor’s
collector. Connect the transistor’s emitter to
ground, and its base to the speaker’s output signal.
When no current flows though the transistor, the
resistor holds P0 high. But when the radio’s speaker
applies a signal to the base, the transistor acts as a
short and pulls P0 low.
Connect P1 to the radio’s call button so the controller can send an “acknowledge” tone by pulling
the call button low. Pins 2 and 3 control relays that
can turn the camera on or off and take a picture.
Remember to pull the stereo wires through the
holes in the enclosure before soldering!
7. Mount the controller and
switches in the enclosure box.
After the circuit is assembled, put rubber feet on
the bottom of the project board and hot-glue it into
the enclosure. Then install the switches (Figure O).