The circuit in the
diagram was originally designed to monitor an oscillator, but can also
be used as a general-purpose level indicator for a.c. signals. It is
based on a quadruple IC containing four NAND
gates. Only three of the gates are used, making the fourth free for
other purposes. All the gates have a Schmitt trigger input. When a 5 V
supply is used, the Type 74HC132 is recommended; for higher voltage, a
Type 4093. Note, however, that these two ICs have different pinouts. In
the diagram, the differing pins of a 4093 are shown in brackets. The
signal to be monitored is applied to the input of the first gate via
capacitor C1. Resistor R2, in conjunction with the protection diode in
the IC, guards the input to high voltages.
In the absence of a signal, resistor R1 holds the input high so that
the output of the gate is low. When a signal of sufficient strength is
received, the input of the gate goes low during the negative half cycle
of the signal, so that the output of the gate goes high in rhythm with
the input signal. However, the Schmitt trigger converts sinusoidal
signals into rectangular ones, which charge capacitor C3 via diode D1.
When the potential across C3 exceeds the threshold at the input of the
second gate, this gate also toggles. The output of the second gate is
then low, which disables the third gate, which functions as an
oscillator. When the level of the input signal drops, C3 is discharged
The potential across the capacitor then no longer exceeds the threshold at the input of IC1b, whereupon IC1c is enabled and the LED flashes The LED
may be connected as shown or as indicated by the dashed line. As shown,
the diode remains off when there is an input signal of sufficient
strength and begins to flash when the signal fails or its level drops.
When the diode is linked to earth, it is on continuously when there is
an input signal, and begins to flash when the input drops. When a 5 V
power supply is used, R5 = 1 kΩ, and the circuit draws a current,
including that of the LED, of 3 mA. The
frequency of the input signal may lie between 10 Hz and 10 MHz. When a
9–12 V supply is used, the value of R5 must be altered as necessary.
Owing to the 4093 being slower than the 74HC132, the upper frequency
of the input signal is then limited to 3 MHz. When the wiper of P1 is
at the level of the supply voltage, the response threshold, USS, lies between 3.5 V (when Ub =5V) and 7 V (when Ub =12V). When the wiper is moved away from the positive supply line, USS
(max) is 1.5 V (when Ub = 5 V). The response threshold is quite
precise: a drop in the input signal level of 50–100 mV is sufficient to
disable the input. When the input level is too high, a preset across the
input terminals enables the level to be reduced to a value that lies in
the desired range above the response threshold.