Illuminated doorbell press switch circuit
This circuit allows you to add an
illuminated doorbell press switch to a
new or existing wireless chime setup.
You can also use it to upgrade an existing illuminated press switch to longlife LED lighting. No more costly and
hard-to-get specialised light bulbs for
your favourite bell switch!
Four 3mm super-bright LEDs of
any colour are connected in series
and housed within the body of the
pushbutton switch. They should be
arranged to shine through the switch
sides; in my case, a Friedland D534
Lightspot. The LED current is limited
to a safe level by the 150W 2W resistor, although the resistance of the wires
leading to the switch has an effect too,
depending on their length.
When the button is pressed, the
LEDs are effectively shorted and the
voltage at the inverting input of the
LM393N comparator (pin 2) drops
below the reference voltage at the
non-inverting input, set by trimpot
VR1. This causes the LM393N’s output
transistor to turn off, switching on NPN
transistor Q1 due to the 10kW pull-up
resistor and energising the small relay.
The normally-open contacts of the
relay can be used to adapt a wireless
doorbell transmitter by bridging its
pushbutton switch, or simply used to
control a bell or chime in a conventional wired doorbell.
The circuit can be easily constructed
on a small piece of veroboard, powered
from a small 12V plugpack and housed
in a small plastic case.
The only setup required is the
adjustment of VR1. With the circuit
wired and powered up, the LEDs in
the pushbutton switch should be illuminated. First measure the voltage at
pin 2 of IC1 with a DVM, then monitor the voltage at pin 3 while adjusting
VR1 until it is about 1V lower.
The exact setting is not especially
critical, but setting it too low can
give trouble if the pushbutton switch
contacts become more resistive due
to corrosion. Setting the voltages too
close can produce unwanted spurious
chime operation.
David Worboys,
Georges Hall, NSW. ($70)
Reading three digital signals with a two-channel oscilloscope
I needed to watch three different digital signals (SPI chip select, clock
and data) but only have a two-channel analog scope.
So I came up with the idea of using resistors to mix the clock and
data signals, as shown, then feeding the combined signal into one of
the scope channels.
As you can see from the scope grab, it works surprisingly well – the
clock pulses ‘ride on’ the data pulses. You can identify the clock pulses
and see whether the data bit is high or low during that pulse. The other
channel is free to be connected to the chip select line, so it can be used
as a trigger to capture the SPI packet.
You could change the value of the resistors to suit the job; 22kW is a
reasonable middle ground as it will not overly load digital signals while
still providing reasonable signal integrity to the scope for moderately
fast signals (up to a few MHz perhaps).
John Rich, Petersham, NSW. ($60)
74
Silicon Chip
Australia's electronics magazine
siliconchip.com.au
