Introduction
In my house there is an inglenook fireplace which
houses a wood burner.
Ordinarily the fireplace is quite a dark place
being recessed and I wanted to provide some lighting to take the darkness away
and to light up the natural stone.
After searching for lighting and considering the
powering options I decided to install an LED lighting string. These strings
come in various sizes and can be cut to length. Powered usually by 12 volt
power supply I wanted to hide as much cable and components from view.
Looking at various switching options I decided to
design a switching unit based on a Passive Infra-Red (PIR) detector and power
transistor to switch the 12 volt LED string on and off.
The main challenges to address are that the PIR detector alone can
only switch low power circuits and the PIR output is limited to a short 'on'
time once activated. What was required is that the PIR detector should operate
with a single hand gesture with circuitry to switch the LED string on, and with
another hand gesture, the PIR switches the LED string off. The PIR module will be mounted out of sight
underneath the inside of the fireplace recess, so a single hand wave will be
sufficient for the module to detect movement. One wave will switch the LED
string on, with another wave switching the LED string off.
The circuit below achieves what I required.
 |
| Passive Infra-Red LED string switching controller |
Circuit
description
The PIR module detection sensitivity is set so
that it is triggered when movement is as close as possible to avoid spurious
activity. The 'on-time' is set to its shortest time, as all that is required is
a single pulse output from detected movement.
The PIR module output is fed to a CMOS dual D-Type
Flip-Flop. Only one Flip-Flop of the CD4013 is needed in this example. The
Flip-Flop is configured so that when the PIR output changes from Low to High,
the output of the CD4013 (Q) goes High. When the PIR output returns to Low
condition, the Q output of the Flip-Flop remains High. Only when another
transition from Low to High on the PIR output takes place will the Flip-Flop
change its output from High to Low. In effect the CD4013 Flip-Flop is
performing a divide by 2 operation, but in this case its output can be used
effectively as a latch.
The output from the CD4013 is fed to Q1,
MOSFET RFP12N10L. The MOSFET is switching
12volts to the LED string, and with the length of string I needed, the total
current draw was approximately 0.5 Amp. The MOSFET is working well within the
operating specifications and does not experience any rise in temperature, which
is important as I chose to mount the circuitry in a sealed aluminium box.
I included the LED (D1) as a switching indicator.
Should the LED string fail to operate and D1 is on, this will indicate that the
LED string has failed.
The PIR module and CD4013 require a 5v supply. The
power regulator 78L05 provides this and is capable of supplying up to 100mA.
Pins 8, 9, 10 & 11 of the CD4013 are taken to
0v. [not shown on the circuit diagram above]
Operating
currents
In operation, this circuit draws a relatively
small amount of power; indeed when inactive, current draw is almost zero.
The following total current measurements were
taken:
PIR inactive: 51uA
PIR active: 0.375mA
CD4013 inactive: 0
CD4013 active: 41uA
LED strip off: 51uA
LED strip on (3 metre length): 400mA
LED1 on draws 1.6mA
References
/ data sheets
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