## Monday, August 20, 2007

### Voltage regulation for toys

It is possible to use a range of voltage values as power sources for circuit bent toys. This approach involves making up the circuit, as described. The total cost for the components is around \$2.50.

This method presents the following advantages:
• The ability to use a rechargeable 9V (or other voltage) battery as a power source
• The ability to have a common ground path for the toy as well as any circuits that may be controlling it.
The circuit itself makes use of a LM317T voltage regulator and has been designed using the information from the datasheet. However, there are a number of limitations / considerations:
• Never use a mains power adaptor with a circuit bent toy. This holds true even if you are using a regulator such as a LM317T.
• The input voltage should be at least 1.5V above the desired output voltage.

1. Set your multimeter to measure low to medium DC voltages. This can be done by locating the DCV section on the multimeter, and turning the dial to the position marked "20" or "20V" or a similar value. This means that the multimeter will be expecting to measure voltages between 0V DC and 20V DC. The display should change to 0.00.

2. Measure the voltage and polarity of the battery terminals on the compartment on the toy that has been / will be circuit bent. First, connect one probe of the multimeter to one terminal and connect the other probe to the other terminal. Write down the voltage that is displayed on the multimeter. Next, determine which wire of the terminal is the positive and which wire is the negative.

This can be done by connecting the red and the black probes of the multimeter to the battery terminals such that the display shows a positive number. The red probe will be at the positive battery terminal and the black probe will be at the negative terminal. If the display is showing a negative number, the red probe will be connected to the negative terminal and the black probe will be connected to the positive. Note the wire that is supplying the positive voltage to the toy and which wire is negative.

3. Identify the LM317T voltage regulator. When looking at the regulator face on, the pin to the left is pin 1 (input voltage), the middle pin is pin 2 (adjust voltage) and the pin to the right is pin 3 (output voltage). Place the LM317T onto the breadboard.

4. Connect the red wire (positive) of a 9V battery snap connector to pin 1 of the LM317T.
Connect the black wire (negative) of a 9V battery snap connector to the blue (ground) bus of the breadboard.

5. Connect pin 3 of the LM317T to the red (power) bus of the breadboard.

6. Connect a 220Ω resistor between pins 2 and 3 of the LM317T regulator. If you are unsure of which resistor is which, consult a resistor colour code chart. Connect a 0.1uF capacitor between pin 1 of the LM317T and ground. Please note that your capacitor might appear different if it is a ceramic capacitor (it will be small, round and orange). In both cases, the polarity of the components are not important.

7. Connect pin 2 of the LM317T to an outer leg of a 1kΩ trim potentiometer. Connect the inner leg of the trim potentiometer to ground.

8. Identify a 1.0uF electrolytic capacitor. The component is shaped like a cylinder with two legs coming out of one end. Unlike the 0.1uF capacitor, this component does have a polarity. The negative side will be marked with a line of minus signs and an arrow pointing towards the leg that is the negative side. Connect the capacitor between pin 3 of the LM317T and ground. The negative leg should be facing ground.

9. Connect the ground (blue bus) to the black probe of the multimeter. Connect the positive voltage (red bus) to the red probe of the multimeter using two alligator clips. Set the multimeter to 20 DCV (the same as step 1). Adjust the trim pot using a small screwdriver until the voltage
on the multimeter read out is equal to or as close as possible to the voltage measured from the battery terminals in step 2.

10. Disconnect / snip the wires that connect the toy to each of its battery terminals. Connect the positive voltage (red bus) to the wire that was connected to the positive terminal of the toy's battery (as measured in step 2). Connect the ground (blue bus) to the wire that was connected to the negative terminal of the toy's battery (as measured in step 2).