THE SMITH-KETTLEWELL VIBRATORY BATTERY TESTER FOR THE DEAF-BLIND
Abstract
We were asked by the Arizona State School for the Deaf and Blind to build a hearing-aid battery tester for hearing impaired students who cannot see well enough to read visual testers. This is a tactile adaptation of our Smith-Kettlewell Auditory Battery Tester, published in Vol. 11, No. 1 of SKTF (Winter, 1990). A 200Hz signal is sent to a small loudspeaker whose vibration can be felt by touching the cone. For those not using hearing aids, consider this a basis for a general-purpose battery tester; it's very good, and if connected to the Radio Shack "Handy Checker," No. 22-096, 9-volt and common flashlight sizes can be tested quite effectively (see SKTF, Winter 1990, for details).
Link to schematic diagram
Configuration
Since button cells come in a wide variety of sizes, we chose not to use "cell holders". A metal plate on the top of the instrument (brass shim stock in our prototype) provides a contact surface for the "can" of the button cell; a flat-head screw attached by a wire is then placed atop the button cell. Standard configurations for buttons use their larger container as a battery positive, with the smaller contact of the cell being battery negative. Therefore, our brass plate is the positive input to the tester and the flat-head screw is the tester's circuit ground.
The box containing our circuit measures 1-1/4 by 2-1/2 by 4-1/2 inches. The square brass plate, measuring 2 inches on a side, is affixed to the top of the box with double-sided Scotch tape; a hole beneath it affords soldering a connecting wire which then goes to the positive input of the tester. A flexible lead comes through a hole next to this plate, with the end of this lead bearing some sort of easy-to-handle probe which can be placed atop the button cell (we used a flat-head screw with two nuts for attachment to the end of this lead).
Along the bottom edge of one side of our instrument case is the on/off switch and a 1/8-inch mini phone jack. A 555 oscillator feeds a 200Hz signal to this jack. We sent the prototype with a small speaker which could be plugged into this jack, the idea being to feel the signal from the cone of the speaker.
The signal from this jack is also able to drive an earphone fairly loudly. Although they cost "an ear and an eye," bone-conduction units make good vibrators (not put against the skull, but felt with the hand). This jack would certainly drive one of those.
How Does It Work?
The duration of buzzes -- the "duty cycle" -- is the indicator of battery voltage. The shorter the buzzes, the better the cell. Long buzzes, with short spaces between, means that the cell is bad. (By exchanging pins 12 and 13 of the LM324, you could reverse the sense of these indications, causing a solid buzz to mean "good," but our experience with an auditory version showed that this mode led to users leaving the tester on and killing its internal battery.)
The cell voltage is "compared" to a calibrated triangle wave. If the voltage of a new cell exceeds the peaks of the triangle wave, the buzzer is not triggered. As the cell voltage drops, more and more of the triangle wave operates the comparator, and bursts of the buzzer get longer as a result. A dead cell, one whose voltage is below the lower peaks of the triangle wave, causes the buzzing to be continuous.
Construction Details
A piece of perforated board -- 2.2 by 3.2 inches -- was cut. Then, corners of one short side were filed away with a rat-tail file so as to accommodate the corner posts at one end of the box. The board was to reside at the right end of the box, leaving room for the 9-volt battery to lie on its edge at the left end.
A small speaker might well fit near the brass contact plate with a hole provided so that a finger could reach through and feel its cone.
Board layout and other construction details are given in the SKTF Vol. 13, No. 2 article. However, that article describes a commercial battery tester as being mounted to the cabinet and the article also recommends a mechanical buzzer to generate the tactual vibrations. The battery tester and the mechanical buzzer are components which are no longer being made.
Circuit Operation
An LM336 voltage standard (said to be a 2.49V standard) feeds a voltage divider so as to give us a standard at half its output value. Fortunately, by feeding its "adjust pin" with a pot, the baseline of 1.245 volts can be shifted to suit our needs. A buffered version of this voltage is created so that an open-drain comparator (called the "composite comparator") can be clamped to this voltage without affecting the standard. (Note that since an FET introduces an inversion of the output swing, the "sense", or polarity, of the composite comparator's inputs is reversed.)
Two op-amps are used to create a triangle wave; one is an integrator being directed up or down by a 2N2222 transistor. The other is part of the composite comparator.
Hysteresis of the composite comparator (adjusted with the 50K trim pot) determines the peak-to-peak amplitude of the triangle; the output from pin 8 of the LM324 goes up and down by the value of that hysteresis.
A simple comparator, the last of four op-amps, compares the voltage of the test cell with the triangle wave. The output of this final stage goes high when positive excursions of the triangle exceed the cell voltage. The 555 oscillator turns on when this output goes high.
Calibration
In the original auditory article, comments refer to how various battery types make the tester behave. For flashlight cells -- zinc-carbon, alkaline, and nicad -- the tester can be set so that its triangle wave is centered at 1.25 volts, with the peak-to-peak amplitude being set to one-half volt. For example, fresh zinc-carbon and alkaline cells will make the tester fall silent; freshly charged nicads will cause the indications to have a 50% duty cycle.
In contrast, zinc-air cells are not so easy to check; their terminal voltage wanders around, making bad ones look okay and good ones look a bit soft. In fact, I disagree with commercial test meters; with those, a battery which was declared dead by a hearing impaired colleague tests fine.
Discharge curves for zinc-air batteries suggest that a plateau of 1.2 volts will be available during most of their lifetime. My initial thought was to set the "offset" pot -- the 10K unit associated with the LM336 -- so that 1.2 volts appears on pin 1 of the LM324 quad op-amp chip. This would mean that 1.2 volts would cause buzzing to occur at a 50% duty cycle -- half on, half off. However, the sample cells I have don't jive with the textbook figures, so I've since set the baseline just shy of 1.18 volts (seen at pin 1).
The "range" pot -- the 50K hysteresis adjustment -- adjusts the amplitude of the triangle wave. (Unfortunately, the frequency of this wave is determined by its amplitude, which is the nature of home-made function generators. It is the duty cycle that is the important indication, not the frequency of buzzes.)
For this latter adjustment, I took a cell that was declared dead and adjusted the 50K pot until the buzzes more or less blended together. Set this way, a fresh cell caused the tester to be quiet.