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Design a light collar for finding pets in the dark

06 Apr 2015  | Vladimir Oleynik

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If you have a pet, you know how difficult it is to find him or her in the dark, especially if your pet has dark fur. The circuit in the figure lets you build a collar with LEDs that helps you find your pet.

The heart of the circuit is the ZXLD381FHTA LED driver from Zetex. Two 1.2 or 1.5V cells power the LED driver, providing the current pulses to illuminate six to eight 20-mA LEDs of any color series. The ZXLD381 dc/dc boost converter's primary application is in LED drivers. A 10-µH inductor helps maintain output voltage higher than the input voltage. Zetex's ZXLD381 data sheet contains hints on inductor selection (Reference).

Figure: This circuit lets you build a collar with LEDs that helps you find your pet.

The circuit contains a light sensor and the LED driver. The LED driver's operation starts when the ambient-light level drops below a certain threshold. When the light level is high, the low collector-to-emitter resistance of the L-7113P3C phototransistor from Kingbright keeps transistors Q1 and Q2 off, and the LEDs don't light. When it gets dark and the voltage at Point A goes high enough, Q1 and Q2 turn on and saturate, providing power for the LED driver.

To power the light collar, you can use any cell or battery with voltage of 1.2 to 10V. Zetex's ZXLD381FHTA operates at 0.9 to 10V at its common-cathode-voltage pin, VCC, but the light sensor's voltage drop slightly narrows it. The LEDs' efficiency decreases when the power supply is at its low limit. If you don't need the light sensor, then omit all resistors, the phototransistor, and both transistors. Meanwhile, the light sensor lengthens battery life, especially if you expect to use the collar frequently. If you use the collar, you can discharge the battery to a voltage lower than 1.2V before discarding it. When the battery voltage drops to 1.2V, connect its battery's positive terminal through switch S1 directly to the LED driver's VCC pin, which bypasses Q1's collector-to-emitter junction with jumper J1. In that case, the light sensor is off, and the minimum power-supply range decreases to 0.9V.

The ZXLD381FHTA comes in a space-saving SOT-23 package. The other circuit components are available in either surface-mount or through-hole versions. The surface-mount versions for Q1 and Q2 are the BC857C and the BC847C, respectively, and the throughhole versions are the BC557C and the BC547C, respectively. The circuit's size does not exceed 0.5 in.2 (12.7×25.4 mm) when you use SMD components and 1 in.2 (25.4×25.4 mm) when you use through-hole-type components. The phototransistor in a through-hole package is easier to see under your pet's fur. Also, it's more convenient to use LEDs with wider viewing angles and larger diameters.

Powering the collar with two fresh AA 1.5V cells in a well-illuminated environment in standby mode consumes approximately 25µA. In a dark environment, the average consumption is about 7 mA. Four Kingbright L-53SRD-H LEDs illuminate the collar.

To improve detection of your pet, you should place all LEDs evenly along the collar's perimeter. You may also need to prevent the pet's fur from overshadowing the phototransistor and LEDs. Your pet may need some time to get used to its new lighting collar. The collar bends in different directions during use, so make it flexible or arrange it as several rigid PCBs with a wired connection.

"ZXLD381, Single or multi cell LED driver solution," Diodes Inc, May 2010.

About the author
Vladimir Oleynik is from Moscow, Russia.

This article is a Design Idea selected for re-publication by the editors. It was first published on October 7, 2010 in

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