Motion Detecting Dog

This article is about making a dog that "blinks" when motion is detected. If you play the first video you will see that the dog's eyes (the two green LEDs) turn OFF when I move my hand behind the sensor. The second video shows all the steps in making the dog. You can either watch the second video or read this article.

Components: 10 mm thickness LEDs (5 mm or 3 mm thickness LEDs might appear to be a bit too small) - 3 (you might need spare), Light Dependent Resistors (LDRs) - 3 (you might need spare), lead free solder, electrical tape, cardboard, transparent sticky tape, 1.2 mm thickness metal wire.

Tools: 60 W soldering iron, circuit holder, pliers, wire stripper, ruler, pen/pencil and scissors.

Optional tools: multi-meter.

The circuit design is very simple. The resistance of light dependent resistor (LDR) drops when the sensor is exposed to light. The smaller is the LDR resistance the higher is the LED current and the brighter is the LED light. When I cast a shadow on the LDR sensor (that you see in the back of dog's head) the LDR resistance increases and the LEDs current falls making it look like they (LEDs) are OFF. The current across each LED is equal to:

Iled = (Vs - Vled) / (Rldr / 2) / 2

(Where: Vs is the 9 V power supply, Vled is the voltage across the LEDs and Rldr is the resistance of the LDR)

The total resistance across two equal resistors is equal to the resistance of each resistor divided by 2.

Rt = R1 * R2 / (R1 + R2)

Since R1 = R2 then:

= R1 * R1 / (2 * R1)

= R1 / 2

I connected to LDRs in parallel to reduce the total LDR resistance and thus increase the LED brightness and increase the circuit sensitivity. According to specifications the minimum resistance of LDR is 2,800 ohms or 2.8 kohms. The minimum resistance of two LDRs connected in parallel will be 2.8 kohm / 2 = 1.4 kohms or 1,400 ohms. Now we can find the maximum LED current.

IledMax = (Vs - Vled) / (RldrMin / 2) / 2

= (9 V - 2 V) / 1,400 ohms / 2

= 2.5 mA

This is a small current. We can increase the maximum current by adding another LDR in parallel or using an LDR with smaller minimum resistance. However, this would cost a lot of money and it is obvious from the video and photos that this current seems to be sufficient to provide a reasonable LED brightness.

Use the wire stripper to strip the battery harness wires.

Use pliers to connect the two light dependent resistors together.

Use pliers to connect the circuit as shown. The longer LED pins should be connected to the positive (LDR) and the shorter LED pin should be connected to the negative (black wire).

Place the circuit into the circuit holder as shown in the photo. Use 60 W soldering iron to solder the circuit.

Insulate the connections with electrical tape.

Cut out the dog body shape. The dimensions are 10 cm by 7 cm rectangle. You then roll the rectangle into a tube. This tube must fit a 9 V battery that will be placed inside. The length of this tube will be 7 cm.

Cut out the dog's head and ears with scissors. Cut out two rectangles. One 4 cm by 7 cm (the head) and another 4 cm by 3 cm (the ears). Use scissors the cut the holes in the middle of the larger rectangle and insert the two LEDs. Now you roll the larger rectangle into a tube leaving the circuit inside.

Draw the ears on the smaller rectangle (4 cm by 3 cm) from the previous step and cut them out. Attach the ears to the head with transparent tape as shown in the photo.

Make the legs from 1.2 mm thickness metal wire. You can cut the wire with pliers.

Make the dog's tail and attach as shown in the photo.