This compact four-digit digital counter is suitable for any application, such as clocks, frequency meters, digital voltmeter, tachometers, stop-watches, scoreboards, etc. The counter module provides a reset switch and a carry output signal.
The maximum count of the module depends on which version of the IC used:
• the MM74C926 pulls its carry output high when counter state 6000 is reached. The IC counts to 9999.
• the MM74C927 is like the MM74C926, but the second most significant digit divides by 6 rather than by 10. Thus, if the clock input frequency is 10 Liz, the display would read minutes, seconds and tenths of seconds. e.g. 9:59.9.
• the MM74C928 is like the MM74C926, but the most significant digit divides by 2 rather than by 10. Also, the carry-out is an overflow indicator that goes high at counter state 2000 and goes low only when the counter is reset. Thus, this is a 3 1/2 digital counter.
All three MM74C92x ICs are CMOS devices. Each has an onboard display multiplexer, which requires only four external switching transistors at the common cathode connections of the 7-segment LED displays. The multiplex frequency is about 1 kHz.
The carry-out terminal may be used to cascade counter modules. Current consumption is of the order of 100 mA.
Finally, it should be noted that clock input signals exceeding the module supply volt, age are not clamped, and should not be allowed to exceed 15 V. The supply voltage range of all three MM74C92x ICs that may be used here is 3-6 V (data taken from National Semiconductor data sheets).
R1-R4:R12 = 1.5 kΩ
R5-R11 = 56 Ω
R13 = 10 kΩ
Capacitors: C1:C2 = 100 nF
Semiconductors: T1-T4 = BC547B
LD1-1,D4 = HD1107
Integrated circuits: IC1 = 74C926/927/928 (see text)
Miscellaneous: S1 = PCB-mount push-button
K1 4-way PCB header