The Science Fair 65-in-1 Electronic Project Kit was introduced in Radio Shack’s 1973 catalog. As its name suggests, it came with instructions for 65 projects. In this article, we’ll build a modified version of project number 29, the Electronic Timer.
In the classic version of this project, a small lightbulb is turned on when the key switch on the Science Fair kit is pressed. Once the key switch is released, how long the lightbulb stays on is controlled by rotating a potentiometer. In the remixed version, we’ll use a microcontroller to control when the lamp turns on. Figure 1 shows what the Remixed Electronic Timer project will look like when it’s operational.
Figure 1. The completed Remixed Electronic Timer project. Image used courtesy of Don Wilcher
This project is an exercise in rapid prototyping of a product concept. As we’ll see, we can build the Remixed Electronic Timer quickly and effectively with just the Science Fair kit, a Renesas microcontroller board and accompanying free project software, and a transistor relay module. Before we discuss our new additions any further, however, let’s take a look at the Electronic Timer project as it appeared in the original manual.
The Electronic Timer Project for the Science Fair 65-in-1 Kit
The manual for the Science Fair 65-in-1 Electronic Project kit includes the following information in each project entry:
- The name of the project.
- A wiring chart.
- A wiring diagram and a circuit schematic.
- An Operation section, which provides the sequence of steps to run the project.
- A How It Works section, which provides a technical explanation of the circuit.
Figure 2 shows the project description page for the Electronic Timer, which is the circuit we’ll be building and then modifying. The goal of this classic project was to demonstrate a fundamental application of the transistor RC (resistive-capacitive) circuit.
Figure 2. [Click to enlarge] The Electronic Timer project in the Science Fair 65-in-1 Electronic Project Kit manual. Image used courtesy of Don Wilcher
The Classic Electronic Timer will be incorporated into our final project, so Figure 2 is a good reference to have on hand. And who could resist the smiles on those cartoon capacitors?
The Remixed Electronic Timer: Project Concept and Operation
When we build our project, we’ll use the following parts of the Science Fair kit:
- Germanium PNP transistor.
- Electrolytic capacitor (×2).
- 50 kΩ potentiometer.
- Signal lamp.
- Key switch.
The electronic parts of the Science Fair 65-in-1 Electronic Project kit are located on a cardboard mounting surface with spring terminals. The spring terminals allow point-to-point wiring.
Apart from the Science Fair kit, we’ll need a Cortex-M microcontroller. I selected the Renesas EK-RA2E1 board, which has a Cortex-M23 core, as an out-of-the-box solution. The board will be connected to the Classic Electronic Timer (the Science Fair kit) by a small transistor relay module.
We’ll use the Science Fair kit’s key switch to test the circuit at the start of operation. After that, we’ll turn the Electronic Timer on by pressing the microcontroller kit’s surface-mount device (SMD) tactile push-button switch. The block diagram in Figure 3 illustrates the device concept.
Figure 3. Remixed Electronic Timer block diagram. Image used courtesy of Don Wilcher
Note that Figure 3 doesn’t include the software portion of the project. The example software for the RA2E1’s interrupt controller unit (ICU) is what allows ON/OFF control using the push-button switch. In line with the original Electronic Timer project’s educational purpose, the ICU software demonstrates how interrupts can act as external triggers to initiate events in switching and control applications.
When the Remixed Electronic Timer is operational, pressing the tactile push-button will toggle an SMD LED. This LED is wired to an R7FA2E1 Cortex-M23 microcontroller’s P915 port pin and will act as an indicator light. The P915 port pin provides an approximately 3.3 V control signal, which will be used to drive the transistor relay module.
The relay module’s Normally Open (N.O) contact will connect it to the Science Fair kit, which will be wired to create the actual timer circuit. The test switch and the transistor relay module’s N.O contact will be wired in parallel.
Figure 4 shows the complete circuit schematic for the Remixed Electronic Timer.
Figure 4. [Click to enlarge] Schematic for the Remixed Electronic Timer. Image used courtesy of Don Wilcher
The left half of the schematic is occupied by the EK-RA2E1 kit and the transistor relay module. Together, these make up what we’ll call the Enable Controller. Note that only the EK-RA2E1 kit’s microcontroller is represented in the figure—the rest of the kit, including the tactile push-button switch, is not shown.
The right half of the schematic is the Classic Electronic Timer. Since we discussed the operation of the Enable Controller earlier in this section, let’s briefly do the same for this portion of the circuit.
When either the key switch (SW1) or the Enable Controller’s push-button switch is pressed, a negative bias voltage is applied to a 2SB germanium PNP transistor (Q1). While Q1 is ON, power is provided to a small incandescent bulb (LA1).
The capacitors (C1 and C2) and the rotary potentiometer (R2) provide a time constant for allowing the transistor—and thus the lightbulb—to remain ON after SW1 has been released or the controller’s tactile push-button is toggled.
The potentiometer is wired as a rheostat (variable resistor). Rotating the potentiometer clockwise increases the resistance; rotating it counterclockwise decreases the resistance. The greater the resistance, the longer the incandescent bulb remains lit.
With the exception of the tactile push-button switch, all of the components named here—SW1, Q1, LA1, C1, C2, and R2—are included in the Science Fair Kit.
Building the Remixed Electronic Timer
The hardware assembly for the Remixed Electronic Timer can be broken down into three parts:
- Wiring up the Science Fair Kit to create the Classic Electronic Timer.
- Wiring the transistor relay module and the Renesas microcontroller kit together to create the Enable Controller.
- Wiring the Enable Controller and the Classic Electronic Timer together to create the final project.
For Step 1, follow the schematic in Figure 4. The classic project instructions reproduced in Figure 2 may also be helpful to you.
That brings us to the Enable Controller. If we refer back to Figure 4, it’s easy to wire the transistor relay module to the EK-RA2E1 board:
- Connect the signal pin to the EK-RA2E1 board’s port pin P915.
- With a jumper wire inserted into the Arduino Uno female header +5 V cavity of the EK-RA2E1 board, attach the opposite end to the +5 V male pin of the transistor relay module.
- To provide a return path for the transistor relay module, insert a wire into the EK-RA2E1 board Arduino Uno female header GND cavity. The other end of the ground wire will attach to the GND pin of the transistor relay module.
- Finally, attach jumper wires to the N.O and common terminal points on the terminal block. The two wires should be wired in parallel with the test switch (SW1 in Figure 4).
The transistor relay module requires +5 V and ground (GND) for proper operation. Figure 5 shows the pinout of a typical +5 VDC transistor relay module.
Figure 5. Pinout of a typical transistor relay module. Image used courtesy of Don Wilcher
The assembled Enable Controller is shown in Figure 6.
Figure 6. The assembled EK-RA2E1 Enable Controller. Image used courtesy of Don Wilcher
Finally, we connect the Enable Controller to the Classic Electronic Timer. The fully assembled Remixed Electronic Timer can be seen in Figure 7.
Figure 7. [Click to enlarge] The fully assembled Remixed Electronic Timer device. Image used courtesy of Don Wilcher
Adding the Project Software
To enable the Cortex-M control feature, we’ll need to install some software for the microcontroller’s interrupt control unit (ICU). The software bundle containing the ICU code for the EK-RA2E1 kit can be found on the Renesas website. Note that you’ll have to register for a free account with Renesas before you can access it.
Once you’re signed up and logged in, navigate to the EK-RA2E1 documentation page and download the application note labeled EK-RA2E1 Example Project Bundle along with its related sample code. In Figure 8, the correct application note is circled in black.
Figure 8. The Example Project Bundle and sample code for the EK-RA2E1 kit. Image (modified) used courtesy of Renesas
To use the ICU code, the Flexible Software Package, which is Eclipse-based, needs to be installed on your microcontroller development machine.
Final Testing and Operation
Once the software install is complete, the Remixed Electronic Timer should be fully functional and ready for us to press buttons. You can view its operation in the video clip below (Figure 9).
