Battery Meter for Pinball Machines

Introduction

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). The 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.

Theory of operation

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 always put out their specified voltage - a small voltage drop is usual.

Tools

A decent soldering iron, a side-cutter, a small flat blade or philips 0 screwdriver, a digital volt meter (DVM) or oscilliscope are all you need.

Schematic

Okay, lets make a start - the circuit schematic:

[Circuit copyright - Clive Jones 7/1996]

339/n = connect to LM339 comparator pin "n"

[Comparator Voltage References]

[Power inputs and decoupling]

[Comparators and LED drive]

Pretty simple eh?

The potential divers - R1+R2, R3+R4 and R5+R6 provide the reference voltages. If you wish to modify the circuit to output different references, then the potential divider voltage can be given by the formula:

5(v) x R2
---------
R1 + R2

Building Instructions

DO NOT FIT THE 339 COMPARATOR UNTIL THE VERY END!

The circuit should fit on a board 2"x 2". Build the potential divders/voltage references first, supply them with +5 volts and ground (don't forget capacitors C1/2) and measure the output - they should be approx the figures given.

Next:- add a 14 pin socket (absolutely vital) to the board and run the reference voltages to the correct pins according to the schematic, then +5 volts and ground - now measure the voltages at the socket to ensure they're present.

Next:- add R7-R9 to the board at the pins specified and link the other 3 ends together.

Next:- Build the following battery failure simulator on another small board then jumper the output to the Vbat line where R7-R9 are joined together:

Next:- trim the 10k linear pot and observe voltage changes at 339/5,7 and 9.

Next:- add the 330 ohm LED current limiting resistors R10-R12 followed by the LED's to the board (or mount them on small flyleads for fitting to the box cover).

Last but not least - add the LM or CA339 quad comparator and power up. You should have the green LED on (exception, battery >4.21v) - now rotate the 10k pot and the LED's should light in sequence - green, amber then flashing red. If they do not then power down and CHECK YOUR CONNECTIONS.

If the circuit works correctly then removed the Vbat sim. and add the 3 flyleads to allow connection to the MPU board of your game. I suggest you use a red wire for +5 volts, a black wire for ground and another wire of you choice for the Vbat line.

Add all the bits into a little box and mount the box onto the backbox wall. I suggest that you actually drill holes in the cover for the LED's to protrude through, or, connect them to the board with small flyleads and glue them into the box cover.

Solder the flyleads to the MPU board with the POWER OFF. Allow the board 2 minutes before soldering in the leads as the onboard capacitors may still hold a charge.

USE COMMON SENSE! - don't tap the +5 volts for the circuit from a chip leg - find a free pad on the board that you can tap from. Wire the battery power from the pack were it enters the board and return to a suitable earth point.

KEEP IT NEAT! - Take pride in you work - make the flyleads tidy - use tye-wraps and lay the leads out nicely in the wiring loom, even better, use an in-line connector so that the remote battery indictor circuit can be disconnected from the board without having to desolder.

Parts Listing

All resistors are 1/4W 5%, all capactors are 50 volt

If you're good with electronics you could add a small piezo electric buzzer for an audible alarm.