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Mobile-ON Alarm/Timer

This little circuit could save your car's battery.

by James W. Elkins, Jr. KA8PHO

At one time or another, we have all walked away from our cars and trucks with the mobile rig still powered up. I have done this on many occasions, t don't like the thought of draining the batter}. I don't like the thought of the radio accidentall> transmitting an unidentified signal, either. The latter could tie up a repeater for hours, or until the culprit is discovered. With these thoughts in mind, I decided that it would be nice to have a control system that would remind me that my equipment was still on. An alarm would do nicely, but another thought that crossed my mind was that I occasionally operate mobile during public service events and so I need to be abie to use the radio gear when the car is shut olT. I decided that a lime delay circuit could be incorporated into the control, to bypass the alarm but still shut itself oft after a predetermined time.

The circuit Lhat I put to work for myself is shown in Figure 1. I decided to call it the "Mobile-ON Alarm/Timer." it is a simple but effective device, easy-to-assemblc and to install with minimum wiring. The completed unit was placed in a Ten-Tec enclosure, painted and tube led to keep its appearance neat in the automobile. Relay K1 is a heavy-duty type used for switching higher current loads. This keeps you from having to run heavy wires to the circuit, so it could be mounted under the hood near the vehicle's buttery terminals. You will also notice from looking ai ihe schematic that I color-coded the wiring. This. 1 thought, would make it easier to trace back any wiring, if the need ever arises. Nothing is more aggravating than opening the hood and finding a massive amount of red and black wires to weed through. Smaller-gauge wiring can be used

Piiofo A, A bird's-eye view oj tiw Mobile-ON A ¡arm/Timer circuitrw

to run back to the contro! box: 18 or 20 gauge should work fine.


Three LEDs are mounted through the front of the enclosure. They provide status indications of the circuit. LED I will light when the vehicle's ignition is on, or while the car is running. Switch SI in ihe ON position will clo^e rela> KL completing the power circuit to the equipment. If the ignition switch is shut off, and switch SI is still in the ON position, an alarm (piezo) will begin to beep and LED 2 will Hash. Returning SI to the center position will shut everything off. If operation of your equipment is desired after shutting off the vehicle, you can place switch SI in the AUTO position and momentarily press S2, a normally-open push-button. Depressing this begins a timing

cycle. The length of the time period that the Mobile-ON Alarm/Timer operates before shutting everything off can be "programmed" by selecting R6. The approximate time delays are provided in the chart with the Figure I schematic. Or, you could instead change C6. These components control the holding time of relay KL LED3 will light while the circuit is in AUTO status. Incidentally. you can also cancel the time delay at any time during the delay period by simply switching to OFF.

As shown in the Parts List, nil of the parts can be easily obtained from your favorite store, catalog supplier, outlet, or surplus pans dealer. The cabinet I used is a Ten-Tec type, number TP-14, but any enclosure that suits your task* will suffice. A small hole was drilled approximately 1/4" to 3/8" in the

Photo B. The finished unit looks presentable in this pointed and table d mend enclosure. 32 73 Amateur Radio Today* June, 1994

Parts List

R2t R3 100* ofim

R4 470fc Ohm

RG Select for time (see Figure 1)

R7 150 ohm

Capacitors [All capacitors 25 or 50 volt ceramic disc type, except C6. which is electrolytic type, 16 volt) C1.C2.C4PC5 0.1 jiF C3 0.01 nF

CG 100 mF

Semico nduclors

LH,U2 NES55 timer

Di, D2, 03. D4 1M4H8 diode LED 1 Green T1 type

LED 2 Red Tl lype

LED 3 Yellow T1 type


K1 Radio Shack #275-226 SPST

12 volt. 30 amp (not On PC board) K2 Radio Shack

#275-241 SPDT 12 voU

PibzO alarm Radio Shack *273-074


51 DPDT center off, toggle.

Radio Shack #275-620

52 Normally open, momentary,

Push button type.

Cabinet TefvTec type TP-H or similar

Color-coded hookup wire In-line fuse hdder

Drilled and etched PC boards wilh component placement si Ik-screened on them are available from Far Circuits, 18N&40 Field Court, Dundee IL 60118 for S3 50 plus 51 SO S&H per order.


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