Introduction: Astable Multivibrator (LED Flasher) With Circuit Scribe

An astable multivibrator is an oscillating circuit between two resistor-capacitor circuits in which the on/off state of both RC circuits is not stable.

That is, two circuits turn on and off with the use of transistors. In this circuit, we demonstrate the state of the circuits with two LEDs that blink back-and-forth!

In this Instructable, I first demonstrate how to build the complete circuit on a breadboard using two NPN transistors and two LEDs. Once that circuit is built, I demonstrate how we can break the circuit out onto a circuit we will draw with your CircuitScribe pen!

Please check out the website I referenced for the circuit:

Here's a video I filmed of the circuit in action:

Step 1: Parts List

Let's first take a look at the parts we'll need to build the astable multivibrator on just a bread board.

You will need …

• 2x 470 Ω resistors (yellow-violet-brown)

• 10x 10k Ω resistors (brown-black-orange) or 2x 47k Ω resistors (yellow-violet-orange)

• 2x 100pF (0.0001μF) ceramic capacitors (marked with 105)

• 2x LEDs

• 2x NPN transistors

• An assortment of jumper wires

• A 9-volt battery and battery connector, or 9-volt power source

• A breadboard

Once you have built the circuit with the above components, we will then draw the circuit with your Circuit Scribe, and assemble the circuit with Circuit Scribe modules.

For that, you will need ...

• 2x NPN transistor modules

• 2x bi-LED modules

• 7x connector modules

• A 9-volt battery module

Step 2: Connect the Resistors

We start by assembling one resistor-capacitor circuit

• If you are using 10k resistors, assemble 5 in series. Otherwise, use just one 47k resistor.

Connect one lead of one 470 resistors and one 10k (or 47k) resistor to the positive (+) rail.

Assemble the rest of the resistors in series to achieve 50k resistance. If you used a 47k resistor, just the one is fine!

I've highlighted the rows that connect the 10k resistors is red.

Bridge the channel in your breadboard with a jumper wire. You'll note that the last resistor is positioned diagonally to accommodate the wire's position.

Step 3: Add the Capacitor

Insert one of the 100pF capacitors with one lead connected to the jumper wire, and the other lead in an empty row.

Note that because this is a ceramic capacitor, rather than an electrolytic one, the capacitor can be placed in either direction.

Step 4: Insert the LED

Insert one LED into the breadboard. It's important to get the polarity correct!

The negative lead of the LED (the cathode) must be connected to the capacitor, while the positive lead (the anode) must be inserted into an unused row.

Step 5: Insert the Transistor

Now we need to insert one of the NPN transistors. Take note of the position of the Collector, Base, and Emitter.

The Collector connects to the negative lead (-) of the LED.

Both the Base and Emitter need to be inserted into unused rows.

Step 6: Connect Some More Leads

Now take a jumper wire and connect the transistor Emitter to the negative rail.

Use another jumper wire to connect the positive lead (+) of the LED to the 470 resistor, and thus the positive rail.

We've just about completed the first RC circuit.

Step 7: Carefully Test the Circuit

We can now test the circuit to see whether the LED lights up.

To test it, connect the base of the transistor to the positive rail.

Now we can connect a power source. I use a set of pin headers, with which I've plugged in a connector. You can do this however you like, as long as the positive (+) end of the battery connects to the resistors, and the negative (-) end connects to the Collector of the transistor.

BE CAREFUL! I didn't correctly account for resistance in this circuit. My transistor burst, started smoking, and hit nearly 300 degrees Fahrenheit! If you connect your power source, I strongly suggest replacing the 470 resistor with another 10k resistor.

Step 8: Starting the Second RC-circuit

If you tested the first RC-circuit in Step 7, remove the lead that connects the Base of the transistor to the positive rail. Disconnect your power source as well.

It's time to build the second RC circuit. This circuit will be a mirror image of the one we've already built.

First, connect the remaining 5 10k resistors, making sure to connect one to the positive rail. Insert a jumper wire across the channel as well.

Step 9: Connect the Second Set of Components

Insert the remaining capacitor with one lead connected to the jumper wire, then insert the remaining LED. Remember, the Cathode needs to connect to the capacitor, and the Anode needs to be inserted into an empty row.

Finally, insert the remaining transistor. The Collector connects to the Cathode of the LED and the capacitor, and the Base and Emitters are inserted into two empty rows.

Step 10: Connect the Remaining Jumpers to the Second RC-circuit

And once again, let's connect the Collector of the transistor to the negative rail, and the Anode of the LED to the 470 resistor.

NOTE: I unintentionally placed the second transistor in the circuit backwards, but the circuit will still work this way. In the first circuit, the transistor Emitter connects to the negative rail.

NOTE TWO: I swapped the 470 resistors with 10k resistors because of the burnt out transistor issue. I also didn't accommodate for the loss of resistance in this circuit, compared to when drawing with the Circuit Sribe. The 10k resistor makes the circuit work correctly.

Step 11: Connecting the Two RC-circuits

Now, we need to connect the base of each transistor to the 50k resistance rail and the capacitor on the opposite circuits.

These are the leads that will allow the capacitors to discharge into the base of the opposite transistor, cutting off the flow of power to the LED in its own circuit. When the capacitor discharges, the LED on the opposite circuit lights up!

Step 12: Connect Power, Watch the LEDs, and Onto Drawing!

Now we can safely connect our power source, and observe the rapid blinking of both LEDs.

And now for the fun part … we get to draw the transistor and LED parts of our circuit.

Step 13: Outline of the Drawn Circuit

The first image is an export of a circuit made with 123D's Circuit Scribe Sketch software. You can reference this image to see what we will sketch out.

1) Start with drawing the six contact pads for the transistors. You don't need to fill the whole circle of your stencil; just enough to get the module feet to make contact. I've labeled the Collector, Base, and Emitter with C, B, and E.

2) Draw the contact pads for the bi-LEDs.

3) Draw the battery terminal contacts.

Now that you have drawn the contacts for the Circuit Scribe component, we have to connect some of the contacts, and draw additional contact pads for the connecting wire components.

4) Draw a line connecting the two transistor Emitters, as well as the negative battery terminal.

5) Connect the negative (-) LED terminals with the transistor Collectors that are closest to each. Draw an additional contact pad that branches off these leads. Note, the Circuit Scribe LED modules are bi-directional, and so can be placed in either direction.

6) Finally, draw additional contact pads that branch off each transistor's Base, the positive (+) battery terminal, and the positive (+) leads for each LED.

Step 14: Place the Circuit Scribe Modules

1) Add the transistors as shown. Be sure the align the Collector, Base, and Emitter correctly!

2) Add the bi-LEDs. Because these LEDs are bi-directional, their polarity is not important. Experiment with both alignments to see the red and blue colors!

3) Add the battery terminal. Be sure to get each terminal aligned correctly.

Step 15: Setting Up the Breadboard for the Circuit Scribe Connector Modules

Now we need to go back to our bread board, and remove the LEDs and transistors, as well as the leads that connect them. Leave everything else!

Step 16: Connect the Positive Leads of the Bi-LEDs to the Breadboard

First, connect the positive leads of the LEDs to the single 10k (originally 470) resistors. I used a white wire for the left circuit, and a red wire for the right.

Ensure the wires are connected to the correct rail, and that the connector is placed on the pad correctly.

Step 17: Connect the Negative Leads of the Bi-LEDs to the Breadboard

Now connect the negative LED leads to the capacitor leads.

Once again, ensure the wires are connected to the correct rail, and the connectors are seated securely on the drawn pads.

Step 18: Connect the Transistor Bases to the Breadboard

Now let's connect the base of each transistor to the positive rail on the OPPOSITE circuit.

Again, mind the position of the wires and the connectors!

Step 19: Connect the Positive Battery Terminal to the Breadboard

Connect the positive battery lead to the positive rail. I used a green wire.

Ensure the wire is connected to the correct rail!

Step 20: Power It Up, and Enjoy the Light Show!


Connect your battery, turn on the switch, and watch your LEDs blink!


Wondering what to do next? Here are some ideas:

• Turn the bi-directional LEDs around.

• Try a larger farad-rating capacitor, or multiple 100pF capacitors in parallel.

• Connect more than two LEDs!