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1-2-5 switching arrangements
Many instruments offer adjustment
in period or frequency range in 1-2-5
steps across a single decade as a knob
or switch is changed.
It is usually followed by a 10-20-50
in the next decade and so on through
subsequent decades. The 1-2-5 relationship makes sense as it approximates a geometrical progression across
the decade with just three steps, with
the step multipliers being 2x, 2.5x
and then 2x.
As the period for monostable and
astable circuitry is determined as a
product of some factor times a resistance, it would be helpful to have a
simple way to create resistance values inversely proportional to these
steps.
This can be done efficiently with
either a centre-off single-pole toggle
switch or a 3-pin header with a single
jumper/shorting block.
With the switch set to centre off (or
the link removed), the only component connected between IN and OUT
is the 5kW resistance. With the switch
in the up (or link 1-to-2) position, the
5kW resistance is in parallel with the
3.333kW resistance, giving 2kW.
With the switch in the down position (or link 3-to-1), the 5kW is in parallel with 1.25kW for an equivalent
value of 1kW.
Sourcing accurate 5kW, 3.333kW
and 1.25kW resistors is not easy. But
if we use a total of 13 resistors of the
same value, we can get theoretically
13 100kW resistors soldered to a toggle
switch, giving 1-2-5 resistance steps.
Simple tripwire alarm
I wanted an alarm that was so simple
that (almost) anyone could use it without instructions. The result is a simple yet versatile circuit that stopped
an intrusion into my car in its first
week of use.
It activates a powerful siren (the
load, between C and D) for 70 seconds
if someone disturbs a wire, ie, if the
circuit is broken between points A and
B. This can be multi-strand wire with
bared ends twisted together, making
it easy to separate.
Alternatively, a piece of string may
be tensioned such that it separates the
wire when flexed.
With the tripwire intact, the charge
Circuit
Ideas
Wanted
92
Silicon Chip
across the 3300μF capacitor is limited
to less than 1V by the current flowing
through diode D1. This is insufficient
to switch Mosfet Q1 on, and Mosfet
Q2 is held off by the tripwire keeping its gate voltage low, so the siren is
not powered.
Once the circuit is broken between
A and B, the gate of Mosfet Q2 is immediately pulled up by the 100kW resistor
and the siren switches on. The 3300μF
capacitor can then charge, and eventually the gate voltage of Mosfet Q1 rises
high enough to switch it on, pulling
the gate of Q2 low and silencing the
siren after about 70 seconds.
Connect points A and B again, and
perfect values by arranging them in
parallel sets.
We can create the 5kW value with
two 10kW resistors in parallel, the
3.333kW value by putting three 10kW
resistors in parallel and the 1.25kW
value by putting eight 10kW resistors in
parallel. As 1% resistors are cheap, the
only real disadvantage of this method
is the space required.
If a single-pole centre-off switch is
used, all these resistors can be soldered to the appropriate terminals of
the switch on a front panel, and this
makes it a simple way to change the
period (or frequency) by a factor of 1,
2 or 5 with just one switch.
Fewer resistors can also be used if
you’re willing to accept slight errors.
For 3.333kW, you can use 3.9kW in
parallel with a 22kW (an error of just
0.6%). For 1.25kW, we can use 1.5kW
in parallel with two 15kW resistors,
which is an exact match.
You can also scale all the resistor
values by the same amount, eg, use sets
of 1kW or 100kW resistors instead of
10kW. The accompanying photo shows
13 100kW resistors soldered to a toggle
switch as suggested above.
Barry Moore,
Minto, NSW. ($80)
without a squeak, the circuit is ready
for another round.
In the interests of simplicity, I
wanted an alarm without an on/off
switch or reset switch. If desired, one
may simply clip the circuit onto a 12V
battery, and it is ready to go.
Three possible scenarios are shown
below the circuit diagram. In the first,
the alarm is powered up, the tripwire
broken and the siren sounds for the
full 70 seconds, then times out. The
second is identical except that the tripwire is reconnected before the timeout,
and the siren is immediately silenced.
In the third case, points A & B are
never connected, and power to the circuit is simply switched on to power
the siren for a fixed period.
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Letterbox counter
This circuit starts counting when
someone inserts a letter in the letterbox
at your home or office. It is designed
to save you time from going to the
letterbox to check if there are letters
inside. The number of letters present
in the box is indicated on a seven-segment display.
It uses a white LED (LED1), an LDR,
a 555 timer (IC1) in monostable mode,
a 4033 seven-segment driver chip (IC2)
and a few other components.
LED1 and LDR1 together work as a
sensor. The resistance of LDR1 changes
in accordance with the intensity of
incident light on it.
When light from LED1 falls on
LDR1, its resistance is low. So when
the light beam is broken, the voltage
at pin 2 of IC1 is low; otherwise, it
is high.
When a letter is inserted into the
letterbox, it passes between LED1 and
LDR1. This change in resistance provides a triggering pulse to pin 2 of IC1,
generating a short-duration pulse at its
output pin 3.
This pulse acts as the clock input
for the 4033 counter and display
driver, IC2.
The output pins of IC2 are connected to various segments a, b, c, d,
e, f and g pins of the seven-segment
display, with the common pin of the
display connected to ground. Each
segment has its own current-limiting
resistor for consistent brightness.
When a letter is delivered to the
letterbox, LED2 momentarily glows,
which indicates that a letter is
received, and the displayed count
increases by one.
When the counter reaches nine, it
resets to zero and the cycle repeats.
Switch S1 is used to reset the counter
when you fetch the letters.
Raj. K. Gorkhali,
Hetauda, Nepal. ($60)
The gate threshold voltages of Mosfets Q1 and Q2 are fairly critical. To
avoid complications, I chose identical
transistors. Even so, component tolerances may vary. If the alarm does
not decisively turn off, or if current
consumption does not fall to about
0.25mA when A and B are closed,
insert another diode in series with D1
to raise the voltage at Q1’s gate.
Almost any 12V battery may be
used as long as it supports the load.
Q2 can handle loads up to 74W. But
note that a heatsink will be required
for heavier loads.
For driving a standard piezo siren,
it will require no heatsink, as they
only draw a few watts. One could also
switch on a 12V lamp if desired.
This circuit will remain on standby
for about eight months using a small
12V 1.4Ah gel battery. A battery pack
of alkaline AA cells may be used for
a similar period of service.
Thomas Scarborough,
Cape Town, South Africa. ($75)
siliconchip.com.au
Australia’s electronics magazine
September 2021 93
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