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Latching output for Remote Monitoring Station
This simple circuit adds extra capabilities to the 4G Remote Monitoring
Station (February 2020; siliconchip.
com.au/Article/12335). It provides a
way for the Remote Monitoring Station to drive the Opto-Isolated Mains
Relay (October 2018; siliconchip.com.
au/Article/11267).
I wanted to be able to switch an appliance on or off by sending an SMS.
As the Monitoring Station project has
a battery-saving feature, the status of
the Arduino output pins is lost when
the Arduino goes to sleep. This circuit
adds a way to preserve the state without increasing the current consumption very much. However, in my case,
mains power is available so that is not
a significant concern.
This circuit is based on a 555 timer
which is used as a flip flop to switch
the relay on and off. It keeps it in the
last state, even when the Arduino is
in sleep mode.
When the circuit is first powered up,
pin 2 of IC1 is held high via the 10kW
pull-up resistor, while pin 6 is kept
low by a 10kW pull-down resistor. The
pin 4 reset input is briefly pulled low
by the 10kW/100nF RC network. This
ensures that the 555 won’t switch the
appliance on after blackouts or power glitches.
The Remote Monitoring Station
code needs to be modified (as per
the instructions in Ask Silicon Chip,
March 2020) to send the selected Arduino pin high when you want the
appliance switched on. The code also
needs to be modified to send another
Arduino pin high when you want the
appliance to switch off.
The selected switch-on pin connects to the base of the NPN transistor Q1 via a diode and 22kW resistor.
When this pin goes high, it switches
Q1 on, pulling pin 2 of IC1 low and
thereby bringing its output pin 3 high.
This powers the appliance up via the
Opto-Isolated Mains Relay. The 555
will stay in this state when the Arduino goes to sleep.
When the Arduino receives a command to switch off, the other pin going
high pulls pin 6 of IC1 high, bringing
its output pin 3 low, which switches
the appliance off.
The diodes on the inputs isolate the
Arduino from the circuit and ensure
that the circuit will only respond to
logic high output levels.
Geoff Coppa,
Toormina, NSW. ($60)
Alternative switched attenuator for Shirt Pocket Oscillator
I am building the Shirt-Pocket Sized Audio DDS Oscillator (September 2020; siliconchip.com.au/Article/14563) in
a 100 x 70 x 50mm aluminium box, using AA batteries for
power and RCA and binding post outputs.
This circuit shows the switched attenuator I will be using, which is different from the one suggested in the article. It uses a centre-off switch that I already had, which is
smaller and easier to fit securely than a rotary switch. The
ranges are not sequential, which is not ideal, but at least the
middle position has the lowest output level.
The need for the 150kW resistor is debatable, given the
tolerance of the potentiometer resistance, but it does give
an 11.111kW resistance in parallel with 12kW, which is the
exact value needed.
Rick Arden,
Gowanbrae, Vic. ($60)
62
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