The Electronics merit badge requires that the scout learn about three types of circuits and build a project in one type. The three types are audio, digital, and control.
To facilitate scout learning, this web page has partial descriptions of circuits in the three types. Six pages of hand-drawn schematics are available to give more detail, e-mail johnenge@earthlink.net . An electronics-knowledgeable merit-badge counselor can lead the scout to learn about the circuits. Once the scout has chosen a circuit to build, the counselor will give construction advice and debug assistance. A DMM is needed for debug, as low as $6 at Harbor Freight. Once the circuit works, the scout needs to question the counselor about the circuit so that the scout can himself explain the operation.
All these circuits can be built with solderless breadboards available at Radio Shack for $16. These are reusable, only one scout in four needs to make the purchase. All the components are available from Mr. Engelbrecht.
1. Digital A fire-control circuit for an Estes model rocket uses a three-input AND gate. To burn the igniter and ignite the rocket motor, the master-arm button is held down. The chief pushes his button, the range safety officer examines the launch site and range to make sure no one is in danger and pushes his button, and the launch officer counts down and pushes his button. This produces a logic high at the output of a three-way AND gate. The high level propagates through the master-arm button to an LED. If the two-transistor circuit is built up with an igniter, the logic high turns on both transistors and the igniter gets 12 volts, burning it and igniting the motor. The circuit can be tested with a real igniter, without the rocket, in which case power should come from a car battery or rechargeable lead-acid sealed battery.
This project only needs one 4011 quad NAND integrated circuit, which can work with 5V to 15V power. A 12V lead-acid sealed battery can do the logic and provide the amp or so of current for the igniter. The battery needs to be charged up, with a voltage around 13.1V.
If this circuit is used with a model rocket, care must be exercised so that no breadboard wires are pulled out. Check the logic operation up to the LED before connecting the igniter, then disconnect the battery before connecting to the igniter. All personnel must be clear of the rocket when the battery is connected, and an audible warning must be given before the master-arm switch is pushed. After the rocket launches, the chief must disconnect the battery.
2. Digital A digital counter integrated circuit, 74193, is used to make a count-to-15 counter. After the master reset, pin 14, is pulsed high, the clock switch is operated. The IC counts to 15 and lights an LED. Any further clocks return the count to 0000 and the count commences again, or master reset can be done again.
There is a "warning" LED at a count of 13. This is done with a 4-way AND gate looking for binary 13, which is 1101. The 4-way AND is built up from two packages of 4011, and a third 4011 debounces the clock switch.
In all, there are four ICs. This project requires 5V power.
After the circuit operation is understood by talking, the scout needs to mark the schematic with IC numbers and pin numbers so that the breadboarding doesn't become confused.
3. Audio A phototransistor is used with two transistors to make a light receiver. See https://sites.google.com/site/solderandcircuits/home/ac-coupled-light-receiver for schematic.
This project gives an earphone signal when audio-frequency AC light strikes the phototransistor. The light can come from a fluorescent lamp, a TV, modulated LED taillights of many recent autos, or a TV infrared remote control. Or you can build an LED microphone transmitter and talk on the light beam, receiving it with your phototransistor receiver.
If you doubt that you can receive a TV remote with this light receiver, shine a TV remote (push any button) from three feet toward a digital camera. Look for the infrared LED to show up on the view screen of the camera.
4. Audio Oscillator uses the most popular analog IC, a 555 timer, to make a variable-frequency oscillator that has variable volume in a speaker or into computer speakers.
5. Control A DC-coil relay is used to make a thermostatically controlled chamber. A 10-cent thermistor senses the temperature in the chamber (a cardboard box). A 25W 120VAC lamp heats the box. When the thermistor gets to about 87 degrees F, a three-transistor circuit pulls in the relay, which is wired to shut off the lamp. The box will be maintained at about 87 degrees F as the circuit cycles the lamp at about twice a minute.
Since this circuit uses 120VAC wall power, it must be wired by a scout with close supervision by a knowledgeable adult. It must be plugged into the wall only with such supervision. It is safest to do this in a kitchen or bathroom that has a GFCI. There is electrocution hazard with this circuit. Repeating in black type: There is electrocution hazard with this circuit.