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R4 and zener diode D2 form a 6.20 VDC voltage reference. U1C buffers the voltage reference, and sends it to the voltage divider. Resistors R5 through R18 form the voltage divider. The buffered voltage reference is "divided" or stepped down. Each op-amp receives a slightly different voltage.

Diode D3 forms the display. D3 is a ten-segment LED array. Each LED is driven by an op-amp. Each op-amp has two inputs:

1. Voltage reference—This is a steady unchanging voltage. Each op-amp receives a slightly different voltage from the voltage divider (R5 through R18).

2. Monitored voltage—This voltage is common to all op-amps. It is a sample of the monitored battery voltage. If the monitored voltage goes up, so does the input to each op-amp.

Each op-amp compares these two voltages. If the monitored voltage is higher than the voltage reference, the op-amp's output goes high. A high output turns on the LED. Each op-amp receives a different voltage reference. This turns on the LEDs at different voltage levels. The voltage divider is set up so that the LEDs turn on in 0.25 VDC steps, covering a range of 12.25 to 14.50 VDC.

Diode D4 is connected opposite of D3(1). If the voltage is less than 12.25 VDC (D3(1) off), D4 will be activated. A blinking LED was chosen for this application.

Breadboarding

Breadboarding can be a rewarding undertaking or a frustrating mess. Take your time. Focus on a single circuit block at one time when building a new circuit. Build the block. Test the block. When the block functions correctly, add the next. This project consists of six functional blocks, as described above.

The following steps were performed when I built the prototype. Due to space limitations, the steps are rather general. See the photo on page 60 to get an idea of parts placement.

1. Install D1 and R1.

2. Connect your power supply to the breadboard. Connect ground to the bus at the bottom of the breadboard. Connect B+ (old radio word for positive supply) to the anode of D1.

3. Apply power, and test with a volt meter. Voltage on the upper bus should be approximately 0.6 VDC less than your power supply voltage.

4. Install D2 and R4 to complete the voltage reference section. This block of the circuit is found on the left-

Homebrew hand side in my example. When complete, you should read about 6.20 VDC on the cathode of D2.

Install R2 and R3 to complete the "monitored voltage" block of the circuit.

12. Monitor the output of the "comparing" op-amps. U2 pin 8 should read approximately 0 volts. U1 pin 7 should read approximately 13 volts.

13. Complete the circuit by connecting the LEDs.

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