Home > Circuits > Up/Down Counter
Modified:20:41, 22 October 2013

Note: Common cathode high brightness displays


The switches to make the counter count up or down are both connected via diodes D1/2 to a short delay network formed by R5/6,C2 of about 120us. This ensures that the U/D input has stabilised in the counter ICs before the clock pulse arrives. This then triggers IC1c/d, 2-input NOR gates, which form a monostable circuit. This provides debouncing of the buttons when clocking the counters by producing a pulse of around 150ms to the clock inputs.

The bistable formed by IC1c/d sets the direction of the counters. The up/down buttons set or reset the U/D signal before the final clock signal reaches the counters.

The remainder of the circuit consists of the counters and decoders. They are connected in synchronous mode - not that it really matters - and the outputs from the counters are decoded by IC3/5. No special features of the decoders are used such as Strobe, Ripple Blanking etc.

At 6v with red high brightness displays, 220ohm resistor gives about 18mA per segment. This circuit will drive up to 1" high brightness displays really well.

The outputs from the decoders can drive about 20mA each and at 6v these means the displays can be driven directly but at higher supply voltages this is likely to exceed the ICs power rating.

To increase this current rating for larger displays and higher supply voltages (up to 15v), one of the following methods can be used:


The most basic type is an NPN It is wired as a switch and saturates when the input rises above about 0.7v. Because of the low volt drop across the transistor, (Vce) very little power is dissipated.

Most low power types can drive up to 200mA or so with gains (hFE) of about 200-300.

If more power is required then another high power transistor can be added (TIP31C) to form a Darlington Pair where the second transistor takes most of the power and the first provides most of the gain.

Alternatively a Darlington package can be used instead - such as TIP122 or TIP142 for up to 5Amps!

For currents up to 500mA, a Darlinton driver array can be used where up to 8 Darlington pair drivers are housed in an 18-pin DIL IC package. These are ok if you are using medium power displays, but the drawback is that you must not exceed the maximum IC package dissipation. Also if one driver malfunctions, you have to replace the whole IC.
NOTE: In all of these modifications, the displays used will have to be configured as Common Anode, not common cathode.

Designed and Written by Phil Townshend 2009
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