Q&A
123
Figure 7
1 = Signal input
2 = +5V
3 = GND
+5V
2k
Amp
MIC
10k
Sound Card Microphone Input
Push On/Off
Switch
current-limited via the 2K resistor so
that plugging in a standard (
nonsegmented) plug won’t short out the
power source. Although the sound
card Mic voltage is spec’ed at 5 volts,
it’s often any voltage between 3 and
5 volts.
Now, if you can fabricate an
adapter cable that matches the plug
of the PC sound card to that of your
headset, you will have a working
system.
Q. I need a simple
circuit to turn a
relay on and off with a
momentary push button.
It needs to turn the
relay on with one push
and off with another
push. The system it
controls will be operating
at 12 volts. The push
button is part of a
decorative piece and
cannot be replaced
with any other buttons
or switches without
severely detracting from the aesthetics,
so replacing it with a push on/off
mechanical button is not an option.
Ray
via Internet
When power is first applied, the reset
line is held low by a 1 µF cap. This
forces the complement output high
so that the relay is in the off state.
The 1 µF cap swiftly charges, which
pulls the Reset input high and arms
the flip-flop. JK flip-flops are natural
toggle switches and are often
cascaded in ripple counters for
counting input pulses. So, when you
press the push button, the flip-flop
flips states and turns on the relay via
the driver transistor. Push the button
again and off goes the relay. The
input of the 74HCT73 has a Schmitt
trigger, so debouncing of the switch
is readily accomplished by hanging
an R/C timing circuit (100K/0.22
µF) on the input.
MAILBAG
+12V Toggle On/Off
Relay
33
0.5W
To Controlled Circuit
On/Off
330
Figure 8
+
1000uF
+5V
Figure 9
2N3906
100k
74HCT73
1k
F
o
r
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
On/Off
JQ
CP _
KQ
R
0.22
100k
1N4148
1uF
A. Let me look into my magic bag
of tricks. Ah, here are two
circuits that should fill the bill.
The first (Figure 8) uses a DPDT
relay that uses one set of contacts
for the on/off function and the other
set to control your load. When
power is first applied, the 1,000
µF capacitor charges through the
33 Ω resistor. Full charge is
reached in about 150 mS. When
the push button switch is pressed
(closed), the cap discharges
through the relay coil and pulls in
the relay. This disconnects the
cap from the circuit and switches
the coil over to the 12-volt line,
which latches the relay on. The
330 Ω resistor bleeds
off any charge
remaining in the cap.
When the button is
pressed again, the coil
sees a shorted (fully
discharged) capacitor
and the relay drops
out. This applies voltage to the capacitor,
which recharges anew.
The second
(Figure 9) is built
around a JK flip-flop.
To
Controlled
Circuit
Toggle On/Off Relay
32
Dear TJ,
In the January 2004 issue, Mark
Farrall requested help in designing a
circuit that would be used for both
oscilloscope vertical scale calibration
and scope probe compensation
adjustment. Your suggestion to use a
78L05 for the voltage reference is an
excellent solution for vertical scale
calibration. Unfortunately, using
a 1 MHz square wave for probe
compensation adjustment presents
a few problems.
A typical scope probe has an
internal 9 MΩ resistor, which, in
series with the scope’s 1 MΩ input
impedance and 10 to 25 pF shunt,
creates a 10:1 voltage ratio (Figure
10). At low frequencies, all of the
load current goes through the
resistive divider; at high frequencies,
the current through the capacity
divider swamps the resistive
component. For the above resistor
and capacitor values, the crossover
occurs at about 1 kHz.
Instead of calibrating the resistive and capacitive divider separately
with variable frequency sources,
most scope probe calibrators use a 1
kHz square wave, which has a
Fourier fundamental component
slightly below the corner frequency
and many higher odd harmonics
MAY 2004