54 May 2017
50% duty cycle square wave. As long as R2 sets the pulse
width of IC1 to be less than the period of IC2, the output
pulse of IC1 will fire for every positive transition of the IC2
output; refer to Figure 4.
When the period of IC2 becomes a little less than the
pulse width of IC1, IC1 will act as a 2:1 frequency divider
(Figure 5). The reason for the “little less” is because of
processing delays in triggering the PIC.
The upper traces are the trigger signal to IC1 (the
frequency divider) which is programmed to trigger on the
rising edge of its input, while the lower traces are the
output of IC1. When the pulse width of IC1 becomes
greater than twice the period of IC2, IC1 will act as a
3:1 frequency divider. This scheme can theoretically be
carried out as far as you want. However, the stability of
the one-shot as well as its trigger input will become a
problem as the divide ratio increases. Also, the range
of input frequencies over which the ratio is constant
decreases as the divide ratio increases.
Only with a divide-by- 2 circuit can you approach a
50% duty cycle from IC1. You cannot get exactly 50%
but you can get within 1% with the 555 replacement
as long as the output pulse is > 100 times the
resolution. The same is true using a 555 except that
you will have to use stable components in the timing
Mims Circuit 13
Circuit 13 (Schematic 15) shows a one-shot used to
drive an analog meter for measuring the frequency of the
input pulses. The pulse width should be a little less than
the period of the highest frequency to be measured. The
response time of the meter is used to average the stream
of pulses that it gets.
This circuit shows a capacitor on its input which is
biased to half the supply voltage. Since the other input of
the trigger comparator is biased at 1/3 of VCC, the input
amplitude must be enough to overcome that difference. It
does, however, allow the 555 circuit to be triggered by an
If you need to measure smaller amplitude signals, you
should be able to change the input bias divider so that its
voltage is closer to 1/3 of VCC.
The circuit in Schematic 16 uses a 10 mA meter. The SCHEMATIC 16. PIC 555 frequency meter.
SCHEMATIC 15. Frequency meter.
FIGURE 5. Frequency divider — divide by 2. FIGURE 4. Frequency divider — no division.