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Car Voltage Alarm

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When on a RAYNET event, equipment may be powered from the car battery and if it becomes too discharged, the carís engine wonít start.  This is especially a problem for a car running a talk-through unit.

Using a separate 12V battery which wonít be charged during the RAYNET event means that transmitter RF outputs will fall over time.  Transmitters are dependent on the correct suppy voltage.  A 50W transmitter may drop to 30W and a 10W transmitter may drop to 6W.

The usual solution is to start the carís engine to charge the battery at regular intervals, but how do you know when to charge the car battery?  One method is to set a watch or clock alarm which sounds at set intervals.  Another is to monitor the carís battery voltage with a volt meter.

The advantages of the described circuit are that the battery voltage is indicated and also, a buzzer sounds when the battery should be charged.

Voltage Monitor and Alarm

The device is a box with two LEDs and a buzzer.  A cable from the box connects to the car battery, either via a cigarette lighter plug or a standard RAYNET 12V plug.  When connected, a green LED lights if the input voltage is greater than 11.5V.  A yellow LED lights if the input voltage is greater than 10.5V.  A buzzer sounds if the input voltage is less than 10.5V but sufficient to operate the buzzer, about 2.5V. 

Alarm Voltage

One information source equates battery charge state on a light current load to voltage as follows:  Greater than 11.5V is a charge state of 50% or more, greater than 10.5V is a charge state of 25% or more, less than 10.5V means trouble.  If my own car (diesel engine) hasnít been driven for a few days, the battery voltage with a light load is 12.3V and when the ignition switch is turned on before starting the engine, the current drawn by the glow plugs drop the indication to 11.5V. 

When operating at a RAYNET event, if the device has only the yellow LED on and the buzzer sounds each time you transmit, it is time to start the carís engine and charge the battery.

Device Indications

        Both LEDs are on and the buzzer off:  The normal state.  The device draws about 15mA from the car battery.  The battery is greater than 11.5V.

        The yellow LED is on and the buzzer off:  The car battery is less than 11.5V and greater than 10.5V.

        Both LEDs are off and the buzzer on:  The battery is less than 10.5V and more than 2.5V.  The device draws about 4mA from the car battery.

        Both LEDs are off and the buzzer off:  The device is not connected or the battery is less than 2.5V.

Car Charging Voltage

A carís voltage regulator is typically set to a maximum of 14.7V and the IC will withstand an input voltage of up to 36V.  The buzzer may fail above 24V.


Many RAYNET members will have a spares box and wonít have to buy everything.  The IC costs only 99p.  Almost any NPN transistor will work.  If bought new, the price of the components is £8.35 (in 2012).  A box and stripboard to make the circuit will add to the cost.

Stripboard Construction

Details of component pricing and a stripboard layout can be seen by clicking a link at the left.  A £10 tool to cut the tracks is not necessary.  A twist drill rotated by fingers will cut a track.


You can send an e-mail message to Cleveland RAYNET Group by clicking here.   This will fail if you use web-based e-mail.
Page updated on 09 January 2017

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