Transistor Controlled Switching of an LED Display: A Radio Shack Classics Circuit Remix

Electronics were not the only product that Radio Shack sold in their stores. Many of Radio Shack’s product lines were geared toward hobbyists and encouraged experimentation, imagination, and technical exploration. In the 1970s, Radio Shack also offered the Science Fair^® brand of experimenter kits designed to teach kids and adults about electronics, physics, magnetism, computers, aerospace, and related sciences. 

Radio Shack's Science Fair xx-in-1 electronic projects included a detailed lab manual for building multiple electronic circuit projects such as a crystal radio, strobe light, audible bird, lie detector, sound effects generators, and a door buzzer. The intent of the Science Fair xx-in-1 electronic project kits was to educate adults and kids about the wonders of electronic technology. The learning occurred while building and testing the detailed electronic circuits in the lab manual. 

Project Overview

In this project article, a circuit from the pages of the Science Fair 150-in-1 Electronic Project kit shown in Figure 1 will be presented with a slight twist. The classic circuit will be discussed and illustrated using a solderless breadboard and contemporary electronic components. Further, the circuit analysis and solderless breadboard layout will be introduced using Multisim Live and Autodesk TinkerCAD circuits online modeling environments. Finally, I will incorporate an M5Stack Core microcontroller to upgrade the manual push-button switching operation.

Figure 1. Science Fair 150-in-1 Electronic Project Kit. Image used courtesy of Informit.

On page 40 of the Science Fair 150-in-1 Electronic Project kit lab manual, as illustrated in Figure 2, there is a classic circuit that illustrates the fundamental application of a transistor, that of an electronic switch. The traditional layout of a typical Science Fair electronic project lab manual provided a short technical description and operation of the circuit.

Figure 2. Transistor control switching of LED display lab manual project page. Image used courtesy of Electronique.

The technical description and circuit operation included a wiring diagram and sequence. The technical description used with the electronic circuit was to educate the hobbyist about the electronics field's terminology and symbols. A detailed wiring diagram showing the attachment of electronic components was provided on the right side of the technical description. A wiring sequence showing a text version of the diagram was provided underneath the illustration.

Selecting Modern Components

A compact modern circuit version can be built using off-the-shelf electronic components. The 2SB and 2SC transistors shown on the electronic circuit schematic diagram can be replaced using complementary PNP and NPN transistors. The 2SB component can be replaced with a 2N3906 PNP transistor. The 2SC transistor replacement is a 2N3904 NPN transistor (the complement to the 2N3906). The original 2SB was a PNP germanium component compared to the new silicon 2N3906 transistor. Surprisingly, the 2SC711 NPN transistor was a silicon component.

The 7-segment LED display is a Common Cathode (CC) optoelectronic device. This component with be substituted with an Avago HDSP-5503 CC optoelectronic component or equivalent. Lastly, the electronic circuit was operated by pressing the key that came with the kit. An ordinary tactile pushbutton switch will replace the key used in the original project.

Designing the New Circuit with TinkerCAD

The electronic project build is easily accomplished using the substituted parts and a few discrete resistors. A half-size solderless breadboard can accommodate the placement and wiring of the components using the original electronic circuit schematic diagram.

As illustrated in Figure 3, the AutoDesk TinkerCAD Circuits online platform allows the placement of virtual electronic components on a solderless breadboard. The online modeling platform also allows testing of the circuit completed on the solderless breadboard. Further, this virtual testing feature ensures proper circuit operation before building the physical device.

Figure 3. Electrical wiring diagram created using TinkerCAD Circuits. Image used courtesy of the author.

The solderless breadboard can easily be populated with components and electrically wired, as shown in Figure 4.

Figure 4. A prototype build of the circuit. Image used courtesy of the author.

Designing the Circuit and Creating a PCB Layout Using KiCad

As shown in Figure 5, we can also use KiCad to design the electronic circuit schematic diagram. 

Figure 5. KiCad electronic circuit schematic diagram. Image used courtesy of the author.

The advantage of KiCad is that the electronic circuit schematic diagram can be translated into a printed circuit board (PCB) product layout, as shown in Figure 5. The layout of the PCB can be aided by using the solderless breadboard as a parts placement guide. If desired, an esthetically appealing product can be made by designing a case using CAD and then fabricating it with a 3D printer.

Figure 6. A 3D PCB image of the transistor controlled switching of an LED display project. Image used courtesy of the author.

Explaining the Circuit Operation Using Multisim

The Transistor Controlled Switching of an LED Display project focuses on illustrating the concept of electronic switching. In 1977, when the kit was sold at Radio Shack stores, digital products like computers, calculators, electronic games, and toys, transistors were at their peak usage. The electronic circuit illustrated this fundament aspect of the transistor by turning on a 7-segment LED display. 

As illustrated in Figure 7, the circuit is powered using an attached 9V battery. When the tactile pushbutton switch S1 is pressed, the base of NPN transistor Q1 is biased through R4, a 22K Ω resistor. This causes Q1 to switch from its OFF state to its ON state. When the transistor turns on, this allows current to flow from the collector to the emitter of Q1, and a ground for the LED segment B and C circuits is provided. This current through Q1 will turn on LED1 and LED2 (corresponding to LED segments B and C). Current limiting resistors R1 and R2 (1K Ω) help ensure the LEDs are biased appropriately. 

Figure 7. Multisim circuit schematic of the transistor controlled LED switching circuit. Image used courtesy of the author.

PNP transistor Q2 NPN transistor is biased through resistor R3 (22K Ω) such that it is always in the ON state. The number “1” will be shown on the 7-segment LED display. 

Analyzing the Circuit Operation Using Multisim

We can use a Multisim model to illustrate the circuit’s operation. Multisim provides voltage and current probes where biasing currents and test point voltages can be measured easily (Figure 8). The use of these measurement probes allows the designer the ability to analyze and troubleshoot their circuit designs. There are several circuit analysis modes that the designer can select. In this project, an interactive mode was selected. The interactive mode allows electrical behaviors and functions to be observed through animation features like illuminated discrete LEDs, operating electrical switches and relays, and sound activation of piezo buzzers. Figure 8 provides the probe simulation data for several key voltages and currents.

Figure 8. Multisim circuit simulation of the transistor controlled LED switching circuit. Image used courtesy of the author.

Circuit Analysis Equations and Manual Calculations

We can also analyze the circuit operation manually using basic circuit equations, datasheet values, and Multisim electrical parameters for the transistors. Some of the manual calculation solutions have an error of 5%.

Notes: 

• PR is the abbreviation for probe
• 4.7K Ω (R4) was used in the Multisim model and circuit analysis instead of 47K Ω
• The 0.75 V for V_BE is the transistor base-emitter junction voltage used in Multisim for simulation analysis
• V_JC is the Multisim base-collector junction potential parameter
• I_PR1 was obtained from the Multisim simulation analysis

$$\begin{eqnarray}
I_{R4} & = & \frac{V_1 – V_{BE}}{R_4} \\ \\
 & = & \frac{9.0 – 0.85 \text{ V}}{4.7K \text{ } \Omega} \\ \\
 & = & 1.734 \text{ mA}
\end{eqnarray}$$

$$\begin{eqnarray}
V_{PR5} & = & V_1 – V_{BE} \\ \\
 & = & 9.0 – 0.75 \text{ V} \\ \\
 & = & 8.25 \text{ V}
\end{eqnarray}$$

$$\begin{eqnarray}
V_{PR1} & = & V_{PR5} + V_{JC} \\ \\
 & = & 8.25 + 0.65 \text{ V} \\ \\
 & = & 8.9 \text{ V}
\end{eqnarray}$$

$$I_{R1} = I_{R2} = 8.116 \text{ mA}$$

$$I_{PR2} \approx 2 \cdot I_{R1} = 2 \cdot 8.116 = 16.232 \text{ mA} $$

This last equation neglects the base current of PNP transistor Q2 which explains why the Multisim value is slightly higher.

Automating the LED Switching Using a Microcontroller

This circuit can easily be automated using an M5Stack Core. The M5Stack Core is an ESP32 microcontroller-based unit providing unique robotics, Bluetooth, and Wi-Fi development opportunities. With the M5Stack Core, a control switch can be created to operate the Transistor Controlled Switching on an LED Display circuit with an aesthetically pleasing control unit, as demonstrated in Figure 9. The M5Stack control switch provides a toggle feature where one button can turn on the 7-segment LED display. A second button on the M5Stack Core will turn off the 7-segment LED display.

Figure 9. M5Stack Core controller for the classic Radio Shack circuit. Image used courtesy of the author.

As presented in this project, a Science Fair 150-in-1 electronic project kit was remixed into new electronic circuit formats. This project intends to illustrate that the classic Science Fair electronic project kit circuits can be modified to create unique 21st-century devices. You may watch the video clip of the M5Stack Core and the transistor controlled switching of an LED display circuit in action.