R1 = 2M2 IC1,IC2,IC3 = 555 Timer/Oscillator
R2 = 3K IC4 = SN7400, 74LS00, etc.
R3,R5 = 270 ohm IC5 = SN74107, 74LS107, etc.
R4 = 500K Led1,Led2 = 5mm, Red/Green, ultra bright
R6 = 220K S1 = On-Off switch, push button
P1,P2 = 500K, Trim pots
C1 = 1uF, 10V
C2,C3 = 4uF, 10V
This is a more advanced unit with a precise timed testing procedure.
R1 (2M2) is same as 2.2 MegOhm.
The two timers determine the allowable accuracy for the timer IC under test. Potentiometers P1 and P2 permit ready
adjustment for the desired range.
With power applied, all timers switch to the high state and begin their cycles. The output of IC1 inhibits the
flip-flop for the interval T1. At T2, the output of IC2 goes low and inhibits any signal from the timer under test.
The period between T1 and T2 is the time alloted for IC3, the timer under test, to complete its cycle and produce a
low output. Only during this time can a high-to-low transition from IC3 trigger the Flip-Flop IC5 so that Led 1
(timer ok), which indicates a good IC, lights up. Led 2 lights up when the test is completed.
Although there can be a few milliseconds of contact bounce when S1 is first closed, thereby causing a delay in
capacitor charging, the delay appears across all of the IC's. But since the ratio of delay times among all three
timers is the same, the effect on test accuracy is nil.
How you get to the 5 volt supply power is up to you. Supply power should be between 4.5V (min) and 5.5 (max).
Probably three 1.5V alkaline battery will total about 4.68V because they are never exactly 1.5 V but always between
1.56 and 1.59V or so, and will do the job until the voltage drops below 4.5V. A simple stabilized 5V power supply
would be better choice, or use a 5 volt regulator with a 9 volt battery, works also. Or use one of the little LM317
circuits found on this site. Do not forget to connect power to the 7400 and 74101 IC's (see circuit diagram).
Standard procedure is that if they are not drawn, they are assumed. You can get the LS types (low schottky) which
draw less current.
Note: While bench testing I found that older timers like the µA555 and the MC1455, although good, were
difficult to test. I tried extending the testing period (in seconds) by adjusting trim pot P2 and got an ok from
led2 for the MC1455 but no such luck with the µA555. The LM555 and NE555 testing were excellent. I used
the regular 7400 and not the 'LS' type. I'll do some more testing.
Read the 555 Timer/Oscillator Tutorial for more information of the
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