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Gel-cell Battery MonitorComputer Services of Winona

www.ComputerServicesWinona.com

Assembly InstructionsUse Fig.1 below to determine proper placement of the parts.

1. The IC socket has already been inserted properly for you. All you need to do is solder it in place. DO NOT insertthe IC at this time. It will be inserted after the first tests below.

2. Insert C1, R1, R2, R4, & R5. Then solder and trim the leads.

3. Insert D1 & Z1 as shown in fig.1. Please pay attention to the direction of the band. Then solder and trim theleads.

4. Insert R3 as shown. Then solder and trim the leads.

5. Insert Q1 so the flat side is facing toward R3. You will need to bend the leads for it to fit. Then solder and trim theleads.

6. Insert the LED so that the long lead is in the hole with the square pad. Then solder and trim the leads.

7. For the connection wire, 22 gauge stranded is suggested, for mechanical reasons. Thread each wire up thru thelarger holes in the board to the component side. Strip 1/4 of insulation from the wire and insert it in the appropriatehole then solder (W2 is pos. and W1 is neg..).

Figure 1.

Parts ListC1 .047 ufD1 1N4003D2 LED

Q1 2N2222AR1 20kR2 20kR3 20k Pot.R4 10kR5 330U1 LM339Z1 1N4733 5.1v

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Theory of Operation

R1 and Z1 form a regulated voltage reference, approx 4.3 volts, that does not vary with the battery voltage.This is connected to the + input of comparitor U1. The voltage divider R3, R4 puts a fraction of the battery voltageat the - input of the comparitor. This voltage varies with the battery voltage. When the battery is charged, the -input is above the reference at the + input, and the output of U1 goes low. As the battery discharges, the - input ofU1 drops, while the + input stays constant. Eventually, the - input falls below the + input and the output of thecomparitor goes high. This is the trip point.

When the output of the comparitor is low, it pulls the base of Q1 low, turning it off. When the output of thecomparitor is not low, it is actually open (its an N channel FET inside the chip). R2 is then allowed to supply thebias current to the base of Q1. Collector current flows through the LED and R5, turning the LED on. When U1 isout of the circuit, R2 always supplies bias to Q1, turning the LED on.

The purpose of C1 is to reduce the magnitude of static discharges to a point that they will not damage the restof the circuit. D1 is to prototect the circuit if it is connected to the battery backwards.

Test Procedure

1. With the IC removed from its socket, apply power to the board (13.0v). The LED should be on. If not, removepower and check all connections and component orientation.2. Using the following chart check for the voltages at the listed junctions with IC U1 removed. Then remove power,insert IC U1 (pin 1 towards the LED). Then apply power and check for the voltages.

IC U1 Removed IC U1 Inserted

Q1 (base) to NEG .71v .077vQ1 (collector) to NEG .12v 11.46vQ1 (emitter) to POS 13.00v 13.00vZ1 - R1 to NEG 4.30v 4.30vZ1 - R1 to POS 8.70v 8.70vR5 - LED to NEG 10.61v 11.40vR3 - R4 to NEG 4.80v 4.80vR3 - R4 to POS 8.20v 8.20v

3. Adjust POT R3 so the voltage reading at R3 - R4 to POS is close to 8.20v. With a 13v source, this will give you atrip point of about 11.6v. If you need it to be exact, contact either KB0NKX, WB0NIU or N0JP.