[The following circuit is adapted from battery monitors and testers published in The Smith-Kettlewell Technical File (SKTF). The first appearance of this system was "The Smith-Kettlewell Battery Tester, Where Silence is Golden", SKTF, Volume 11, No. 1, Winter 1990.]

Link to schematic diagram

The application here was inspired by a request from a visually impaired ham-radio operator whose "rig" (transceiver) specifically requires that its supply voltage be above 12 volts under load. When powered by a storage battery (as in emergency operation), his unit would malfunction on transmit, which could not be detected by the operator. Thus, he suggested that a sensitive warning system be devised (other than a multi-digit meter) by which the operator would be alerted to an impending low battery condition. Since the time of that request, we have become interested in providing an alarm for riders of motorized wheelchairs -- an audible indicator that would warn the user that he should put a charge on the battery before "deep discharge" damages it. Thus, the values and adjustments shown here are for voltages below 11.66 volts. Smith-Kettlewell's "Rehabilitation Engineering Research Center", funded by the National Institute on Disability and Rehabilitation Research, devised a battery-checking scheme which has found its way into several of our designs -- a vibratory hearing-aid button-cell tester for the deaf-blind, an auditory adaptation of the Radio Shack "Handy Checker" (see the above byline), and a test instrument for servicing "talking book" players.

The principle of our readout is to generate bursts (audio "beeps", or vibratory "buzzes" for those who cannot hear audio signals); their duration indicates battery voltage. (In this implementation, the output is auditory.)

Specifically, an exhausted battery elicits a steady signal; a fresh battery causes the instrument to fall silent. A "window" (a range of voltages) can be set so that short beeps occur at the top of the range, with beeps becoming longer and longer as the voltage drops, joining together to make a constant tone at the lowest voltage in this selected range.

To accomplish this, the battery voltage is "compared" with a slow-running triangle wave whose minimum and maximum excursions fall within a desired range; this range is the "window" of battery voltages for which the device is to alert the user. Within this range, the comparator will gate the audio oscillator on and off. Thus, when the battery voltage drops below the positive excursions of the triangle, beeps will be produced. Short at first, these tone bursts will become longer and longer as the battery voltage drops, allowing more and more of the triangle wave to enable the oscillator. The instrument is calibrated so that the lowest excursions of the triangle wave coincide with complete exhaustion of the battery.

Circuit Description:

This circuit can be powered from the battery under test. No on/off switch is necessary. "Circuit ground" (for this auditory device) goes to the negative side of the storage battery. The positive of the battery goes through a 1/4-amp fuse to the anode of a protection diode (such as a 1N4001); the cathode of this diode goes to the "VCC line" of the project.

An LM336 2.5V standard has its "anode" grounded; its "cathode" goes through 3.3K to the VCC line. The junction of the 2.2K and the 336 is the 2.5V point. This 2.5V point goes through two 10K 1% resistors in series to ground. The junction of these resistors is bypassed by 1uF (negative of this cap at ground). The junction of these 10K 1% resistors is called the "unbuffered 1.25V point".

An Lm324 quad op-amp has pin 4 going to the VCC line; pin 11 is grounded. Pin 3 of the 324 goes to the aforementioned unbuffered 1.25V point (the junction of the 1% resistors). Pins 1 and 2 are tied together, and thus, pin 1 is a buffered 1.25V point.

Pin 7 of the 324 goes through 47K to the gate of a 2N7000 VMOS FET. The source of this FET is grounded. The FET drain goes through 1.6K to the 2.5V point.

Pin 10 of the 324 goes to the unbuffered 1.25V point (the junction of the 1% resistors). Between pins 8 and 9 is a 0.22uF Mylar capacitor. Pin 9 goes through 1.8 megohms to the 2.5V point. Pin 9 also goes through 910K to the collector of a 2N2222 transistor. The emitter of this 2222 is grounded. Its base goes through 47K to the drain of the VMOS FET.

The drain of the FET also goes through 75K, then through 51K to pin 1 of the 324, the buffered 1.25V line. The junction of this divider goes to pin 6 of the 324. Pin 5 of the 324 goes to its own pin 8.

Pin 8 of the 324, the triangle-wave output, goes through 33K to the top of a 10K trim pot; the bottom of this pot goes to pin 1. The arm of this pot goes to pin 12 of the 324, the non-inverting input of the final comparator.

Pin 13 goes to the arm of a 10K trim pot. The bottom of this pot goes through 10K to ground. The top of this pot goes through 100K to the junction of the protection diode and the fuse.

A 555 timer chip has pin 1 grounded. Pins 4 and 8 are tied together and go through a 10-ohm 1/2-watt resistor to the VCC line. Between pins 1 and 8 is the parallel combination of 0.1uF and 220uF (negative of the latter at pin 1).

Pins 2 and 6 of the 555 are tied together and go through 0.01uF to ground. Pins 2 and 6 also go through 47K to pin 7. Pin 7 goes through another 47K resistor to pin 14 of the 324.

Pin 3 of the 555 goes through an 82-ohm 1/2-watt resistor to one side of the loudspeaker. The other side of the speaker goes Through an SPST toggle switch (a "mute" switch) to pin 8 of the 555. If a flashing LED is also desired, pin 3 of the 555 also goes through a resistor (from perhaps 470 ohms to 1.2K) to the cathode of the LED; the LED anode goes to pin 8 of the 555.


The trim pot off pin 1 of the 324 adjusts the peak-to-peak amplitude of the triangle wave (as seen by the comparator of LM324 pins 12, 13, and 14). This establishes the range over which the audible indications pulsate. With the values chosen here (33K in series with the 10K element of the pot), this range can be as high as 0.4 volts peak-to-peak. In other words, with the wiper of this pot all the way toward the 33K resistor, there will be approximately 0.4 volts difference between the point at which bursts of the tone are just audible ("clicks" in the loudspeaker) and the point at which the tone ceases to be interrupted (the bursts blending together into a solid tone).

The trim pot of the input circuit is part of a voltage divider which is used to set the point at which the indicator sounds with a 50% duty cycle (half on, half off, coincident with the midpoint of the triangle wave). Or, the two trimmers can be juggled experimentally to produce your desired alarm condition.

With a variable bench supply, you can pick both the "span" of useful indications and the voltage at which the device starts alerting you. The author has been advised that it is useful to know that the battery should, if possible, be recharged when its voltage drops to 11.66 volts, and that the user should know that damage to the battery is being done when it drops below 11.4 volts. (I have noticed a significant effect of "loading" the battery, and I assume this lower limit of 11.4 volts should be taken seriously, even under load. Usually, the unloaded battery recovers its "terminal voltage" quickly after the load is removed.)

Some ham-radio sets misbehave if their supply voltage drops below 12 volts. The trimmer sensing the supply voltage can be set to warn the user at this 12-volt level.