Battery Meter for Pinball Machines

Posted on Feb 5, 2014

The circuit is designed to be fitted into a small box and then fitted into your backbox (the exact choice of position is yours). Only 3 wires are required - one for the battery +v, one for the supply +5 volts from the MPU board or power supply and a common return to ground. The circuit is designed for 4. 5 volt battery packs (3 cell x 1. 5 volts). T

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he circuit gives level indication using 3 LED`s - one green (good), one amber/yellow (mid) and one flashing red LED indicating low battery power - 3. 2v or less approx. I think a 1. 3-1. 5 volt drop should be enough warning - I can`t guage the drop-out voltage for CMOS memory standby in WPC games because of the ASIC, so, I`ve gone for what you see here. 3 comparators compare the incomming battery voltage against 3 reference voltages provided by 3 potential dividers (see schematic), the 3 comparators directly drive the LED`s by sinking current to ground (internal). On power-up, the Green LED will always be on indicating good battery power. As the battery voltage drops, the amber LED will light, followed by the red flashing LED, the previous LED`s remain on until all the LED`s are lit. I designed the circuit in this manner to cut down the component count and make the building easier. If you wish to modify the circuit so that LED`s will extinguish one after the other - I suggest you use a 74HC03 quad NAND gate with open-drain outputs to sink the LED current, and, who`s input`s is switched by next comparator in sequence thereby turning the previous gate off (I would have given the schematic details here but I don`t have the IC handy to incorporate into the design at the time). IMPORTANT NOTE: If your battery pack provides 4. 21 volts or greater when new then all the leds will remain off. In most cases, new batteries do not...

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