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SILICON CHIP Mini Projects #017 – by Tim Blythman Automatic Night Light While Arduino modules make it easy to add sensors to microcontrollers, the same modules can often be used without a micro. For example, Jaycar’s XC4444 PIR sensor modules can be easily turned into a night light controller with just a few extra parts. M any night lights are programmed to only turn on when they detect motion in the dark, using a PIR (passive infrared) motion sensor and an LDR (light dependent resistor) to detect the ambient light level. It’s certainly a handy thing to have around the house. Recently, we came across an application note explaining how to add an LDR to the BISS0001 PIR sensor controller chips, as used in Jaycar’s XC4444 PIR sensor module. By adding an LDR to the PIR module, the PIR sensor will only operate when the LDR has a high resistance; that is, when it is dark. We just need a way for the PIR sensor module to switch on a light, and we have a useful circuit. One bonus of this arrangement is that these PIR sensor controllers are designed to have a very low quiescent (idle) current, so our circuit is wellsuited to being powered by batteries. It will only draw any significant current when the light is actually on. We’ll mention some other features of the PIR sensor module a bit later. It is quite configurable and has some adjustments that can be changed to tweak its behaviour. The XC4444 PIR module This module has a PIR sensor with two sensing elements. The elements detect the IR radiation that is passively emitted from people and animals. The elements are arranged so the output is their difference. When a passive IR emitter walks past, the signal changes as the elements report differing amounts of IR radiation. The BISS0001 PIR sensor controller chip mentioned earlier turns this into a digital output signal that is usually low but goes high when a moving object is detected. The module has two trimpots to set the sensitivity and the duration of the output pulse. Figs.1 & 2 show a block diagram and their corresponding parts Fig.1 (left): this block diagram shows what is built into the PIR sensor module’s small PCB. The output has a series 1kW resistor, so we can directly connect it to a bipolar transistor’s base. DELAY TRIMPOT SENSITIVITY TRIMPOT VOLTAGE REGULATOR LDR (NOT FITTED) After removing the plastic lens, the LDR module is soldered to the pads labelled “RL” on the PIR module as shown. The LDR’s light-sensitive side must be facing the same direction as the IR sensor. 52 Silicon Chip PIR SENSOR ON REVERSE OF PCB POWER AND IO HEADER CONTROLLER IC Fig.2 (above): the locations of the parts on the PIR PCB. The two pads near the header are for the optional LDR, while two others can accept a thermistor to provide temperature compensation. Australia's electronics magazine siliconchip.com.au ADVANCED TEST T EEZERS The Advanced Test Tweezers have 10 different modes, so you can measure ☑ Resistance: 1Ω to 40MΩ, ±1% ☑ Capacitance: 10pF to 150μF, ±5% ☑ Diode forward voltage: 0-2.4V, ±2% ☑ Combined resistance/ capacitance/diode display ☑ Voltmeter: 0 to ±30V ±2% ☑ Oscilloscope: ranges ±30V at up to 25kSa/s ☑ Serial UART decoder ☑ I/V curve plotter ☑ Logic probe ☑ Audio tone/square wave on the module. Note that the module does not come with the LDR fitted, so it needs to be added, but that is quite easy. emitter (E) pin. As a result, all three elements of the RGB LED light up, providing white illumination. The human eye is surprisingly sensitive, so even a small LED module like this will provide adequate light in a dark room. Circuit details Fig.3 is the circuit for our Night Light. Power comes from a 3×AA battery holder, providing a nominal 4.5V. The PIR sensor module contains a 3.3V regulator and will work with any supply voltage from 4V to 12V. We have arranged for the output of the sensor to drive an NPN transistor. A bipolar junction transistor like this should have a base resistor to limit the base current, but there is actually a 1kW resistor in series with the module’s output, so an external resistor is not needed. When the output goes high, it biases the transistor on and current can flow from the transistor’s collector (C) to its generator Assembly Because soldering is needed to add the LDR to the PIR module, we decided to build the Night Light on a small prototyping board. The wiring is elementary and could easily be done without a prototyping board, but it makes a nice, stable base for the device. The first step is to fit the LDR to the PIR module. The large plastic lens is just a friction fit to the PIR module’s PCB, so it can be pulled or prised off with a flat-tipped screwdriver. Having done that, solder the LDR to the pads labelled RL on the PCB, with Complete Kit (Cat SC6631) siliconchip.com.au/Shop/20/6631 The kit includes everything pictured, except the lithium coin cell and optional programming header. See the series of articles in the February & March 2023 issues for more details (siliconchip.com.au/Series/396). the LDR’s light sensitive side facing in the same direction as the existing IR sensor. Make sure the LDR doesn’t block the IR sensor; refer to our photos to see how we arranged it. After that, pop the lens back on. Next, carefully bend the three-pin header to allow the PIR module to be mounted facing outwards on the prototyping board. Again, refer to our photos if you aren’t sure about this. The prototyping board has two copper tracks that snake around it, which we used for the Vcc (4.5V) and GND (0V) rails. Fig.4 shows how we laid out the parts and wiring. Note that this assumes you have a −BRG marked RGB LED module, as we got from our local Jaycar. Other versions of the module may be wired differently and may not even need the three external resistors. Fig.3: in our Night Light circuit, a signal from the PIR module drives the transistor which then switches the LED module on. It will run for a long time from three AA cells, using negligible power until it is activated. siliconchip.com.au Australia's electronics magazine December 2024  53 Parts List – Automatic Night Light (JMP017) 1 PIR sensor module [Jaycar XC4444] 1 RGB LED module [Jaycar XC4428] 1 light-dependent resistor (LDR) [Jaycar RD3480] 1 BC546, BC547, BC548, BC549 or similar NPN transistor [Jaycar ZT2154] 3 150W ½W axial resistors [Jaycar RR0552] 1 mini prototyping board [Jaycar HP9556] 1 3×AA cell holder [Jaycar PH9274] 3 AA cells 1 10cm length of insulated wire (cut from excess length on battery holder) 4 self-adhesive feet (optional) [Jaycar HP0815] If you have a part marked +BRG or similar (meaning it is a common anode type, instead of common cathode), the + or anode should go to the red 4.5V supply and the common end of the three resistors should connect to the transistor’s collector. If you want to use a different LED, make sure that it has the appropriate series resistor for your chosen voltage and connect the anode to 4.5V and the cathode to the transistor collector. We recommend fitting the lower-­ profile parts (like resistors) and wires before the modules, as they are difficult to get to otherwise. Fortunately, two of the three transistor leads line up with two of the PIR module’s leads, simplifying the layout. Make sure you don’t get the transistor backwards. Now add the LED module, battery holder and insert the cells. You can test that the LED is wired correctly by shorting the outer two leads of the transistor (emitter and collector), which could cause it to light, as long as the cells are inserted in the battery holder. If all is well, remove the cells, solder the PIR module in place and then refit the cells. As we’ve added the LDR to the PIR module, you’ll need a dark room to test the Night Light. Walk in front of it and check that the LED lights up. It might only be for a second or so, but that is enough to know it is working. Conclusion Using the two trimpots shown in Fig.2, you can adjust the sensitivity and delay time. Both increase when the trimpots are rotated clockwise. The delay refers to the time that the output is high and thus the time the LED is on after each trigger event. We suggest setting the delay to its minimum (fully anti-clockwise) and sensitivity near the middle, then adjust the sensitivity until you are happy with how close you have to get before it’s triggered. That’s easier to do when the LED only stays on for about a second at a time. After that, adjust the delay to your liking. The working range is about one second to three minutes. Fit rubber feet if you wish, and set up the Night Light where it is needed. We measured the current draw of our prototype at 50μA when idle and 35mA when the LED was on, so the battery life will mostly depend on how much the light is activated. If the Night Light is used infrequently, the AA cells should last for several years. You will see the light getting dimmer as the battery SC goes flat. Fig.4 (left): a top view of how we laid out our prototype. The copper pattern is not visible from this angle; we have shown it here so you can see how the 4.5V and ground rails are connected on the other side of the PCB. The photo above shows how we have laid out the wiring on our prototyping board. The adjacent close-up photo shows the two points under the board where we used blobs of solder to connect to the 4.5V supply and ground rails. 54 Silicon Chip Australia's electronics magazine siliconchip.com.au