Retro Prototyping, Great for Teaching

Have you ever wondered where the term "Breadboard" came from? Here's an example of what breadboards were all about. In the early days of electronics, components were large and cumbersome. They didn't have transistors or integrated circuits, only vacuum tubes. So it was common practice to build prototype circuits on a block of wood using nails or screws as circuit tie points. Tube sockets could be screwed down with standoffs, transformers and larger components were also screwed to the board. Resistors, capacitors and coils could be soldered to nailheads.

This technique is still useful for some circuits. This is an example of a project I had for kids who wanted to learn electronics. They could build the circuit, following a schematic. When finished, they could take the circuit home and keep it. It did not have to be disassembled for the next user, as is the case with modern solderless breadboards.

The circuit here is a simple astable multivibrator. The red and green LEDs alternate. The flashing rate is determined by the RC time constant of the resistors and capacitors.

1. A piece of soft wood about 3 by 5 inches (or larger). Clear pine work well.

2. Some spray adhesive.

3. Some 3/4" copper plated weatherstrip nails (available at Home Depot).

4. About a foot of 24ga tin plated buss wire. (or strip the insulation off 24ga solid wire)

5. Two resistors (R1 and R4), 470 Ohms, 1/4 watt.

6. Two resistors (R2 and R3), 51,000 Ohms, 1/4 watt. (see text)

7. Two capacitors (C1 and C2), 10uF Aluminum Electrolytic. (see text)

8. Two 5mm LEDs, one red and one green is good.

9. Two NPN Bipolars small signal transistors. 2N2222, 2N3904, or equivalent.

10. A 9 volt battery and battery clip.

1. Small hammer.

2. Soldering iron and solder

3. Scissors.

4. Wire cutters.

5. Needle nose pliers.

6. Eye Protection

Download the PDF file and print it. Be sure to choose "actual size" when printing. The resulting image should be about 3 inches wide and 2 inches high.

Cutout the schematic. Spray the back very lightly with spray adhesive. Press the schematic on the board, roughly in the center.

From now on, be sure you wear eye protection!

Using a small hammer, drive the nails into the board at each round dot on the schematic. These point are known as circuit nodes. There are 14 nodes.

The nails should be driven in approximately 1/4". This would leave 1/2" above the board.

Note: When working on things such as this, it is easiest to start in the middle of the board and work to the outside. The same is true for soldering.

If you look closely at the schematic, you will see several nodes that are connected with wire. The ground buss along the bottom of the circuit, the power buss along the top of the circuit, and the two connections from the capacitors to the base of the transistors.

Install each of the four wires by snugly wrapping the wire around a nail then a wrap around each nail it connects to. In the center of the circuit, the two jumpers must cross each other, but they must not touch. Install the first of these close to the board. Install the second high on the nails, just below the head.

Solder each wire to the side of the nail and trim off the excess wire. Be care, don't let the wires fly across the room.

Your breadboard should look like the one in the photo.

Solder a blob of solder on the top of each nail. If the leads (pronounced leeds) of your components are clean, the small blob are all the solder you will need. Look at the close-up photo. Do your nails look like this? Notice the solder of the jumper wire and the blob on top of the nail.

There are four resistors to solder in place. On this prototype, we are using the "clean" approach. This is where the parts are trimmed closely for a neat appearance. The other approach appears sloppy because the leads of the components are left long to allow later reuse. More about this later.

R1 and R4 are 470 Ohms or Yellow-Violet-Brown-Gold. Lay the resistor across the nail heads. Solder each end by simply re-flowing the solder blob. Trim each lead close to the nail head.

R2 and R3 are 51K (51,000) Ohms or Green-Brown-Orange-Gold. Solder and trim.

Actually the value of R2 and R3 can be different if you would like to use different value capacitors or change the flashing rate. I made the schematic, then found I didn't have any 10uF capacitors on hand. So I used some 22uF capacitors and used 27K resistor instead. This gives approximately the same flashing rate.

The LEDs must go in a certain way. Notice the schematic shows an arrow with the point touching a bar. The bar is the cathode, the arrow is the anode. If you look closely at the LED you will see one lead is longer (the Anode) and there is a small flat spot on the LED body near the shorter lead (the Cathode).

Carefully bend the each lead of the LED 90 degrees as in the photo. It is best to use the needle-nose pliers to hold the lead near the body of the LED, then bend the lead from the open end. This prevents the bending action from cracking the body of the LED.

Solder each LED with the flat side toward the transistor. Trim both leads.

These are Aluminum Electrolytic capacitors. They also have to be installed in a particular way. Like the LEDs, the longest lead of the capacitor is the "positive" side. You will see the opposite side marked with "-", the minus sign.

Bend the leads in the same manor as the LED leads. Solder in place minding the "+" mark on the schematic is the longer lead. Trim after soldering.

Transistors have three leads, Collector, Base, and Emitter. Look closely at the photo. Notice the flat side of both transistors is on the right side, even though the center lead is bent in a different direction. On Q1, the center lead is bent toward the flat side, on Q2, it is bent toward the round side.

Solder each transistor in place and trim.

Solder the red (positive) wire from the battery clip to the power buss at the top of the circuit. (top of R1)

Solder the black (negative) wire to the ground buss, (bottom of Q1)

Plug in the battery. The LEDs some flash back and forth, see the video as an example.

You can use a few nails to hold the battery in place if desired.

Here's another breadboard. Notice the leads of the components have been left at full length. It's not pretty, but it does allow the board to be taken apart and the parts used for other projects. The problem with this is that it is easy to short-circuit leads together.