You’ll use this circuit as a kind of musical instrument whose pitch changes depending on how close you get to a simple light sensor.
Meet the 555 Timer
The 555 has been used in everything from toys to spacecraft. It can make lights flash, activate alarm systems, put spaces between beeps, and create the beeps themselves.
It is the most successful chip in history, both in number sold (tens of billions and counting) and its design’s longevity (largely unchanged for ~50 years).
Spread the contents of your breadboard kit onto your tray.
Find the little bug-like chip. This is a special integrated circuit called a 555 timer. Each leg is a “pin”. Each pin has a function you can learn about as you continue exploring electronics. It’s not important to understand it all now, but you’ll need to know each pin’s number.
When the half-circle indentation is on the left, so that the state of Texas in the logo has north side up, then pin 1 is the lower left pin.
The dot always marks the pin 1 on a chip, and the other pins are numbered counter-clockwise: 1, 2, 3, 4 along the bottom, and 8, 7, 6, 5 along the top side.
1 - 555 Timer
Put your 555 timer into the board without bending its legs. It matters which way you connect the other components, so make sure the small half-circle indentation is on the left, so that the state of Texas in the logo has north side up.
The 555 timer should bridge the two halves of the board and fit snugly. Push it all the way into the board!
(We used holes 20e to 17f for our chip.)
2 - Jumper
Prototypers use short pieces of wire called jumpers to connect different rows and/or buses in breadboards. You could use any kind of wire. Sometimes people even use staples!
Every jumper behaves the same regardless of its color.
Let’s start with pin 8, the one in the top left corner of the 555 timer if the dot is on the left. Add a jumper from pin 8 to the red (+) power bus (any hole along the long red line at the top).
To connect to pin 8, you can use any of the four holes above that pin. All five holes (20a-e) are connected together. (We used hole 20d.)
Make sure you firmly push components all the way into the breadboard, as if you are pinning a poster to a bulletin board.
3 - Resistor
Resistors lower the amount of current going through a circuit. They are often used to protect other components in the circuit.
Note that there are at least two resistors in your kit with different bands of color on them.
To install your first resistor, find the 10kΩ one with bands colored | | | | brown, black, orange, gold. Use it to connect from pin 7 (we used hole 19a) to the (+) power bus (any hole along the red line at the top).
This resistor controls the pitch. The lower the value of the resistor you use, the higher pitch you’ll get out of the speaker. For example, a 180Ω (brown, gray, brown) resistor works well with this speaker, but the sound is way more annoying!
4 - Photoresistor
Photoresistors’ values change in different brightnesses of light. The brighter the light they sense, the lower their resistance value.
Use the photoresistor to connect pin 7 (the same pin that has your first resistor) to any hole in the lower left.
(We used holes 19d and 30i.)
5 - Capacitor
Capacitors store electricity for short amounts of time, typically fractions of a second. They are often used to smooth out fast changes in current. A ceramic one like this one can be inserted in either direction.
Notice your yellow 0.47μF capacitor has short legs.
You may want to use a tool to bend right angles in each leg before placing it.
Use the capacitor to connect from the row you just used for the left leg of the photoresistor (we used hole 30j) to the (–) ground bus (any hole along the blue line at the bottom).
6 - Jumper
Use a jumper to connect that same row (holes 30f-j) with your photoresistor and capacitor to pin 6.
(We used holes 30f and 18d.)
Nothing is connected to pin 5.
The next pin you’ll use is pin 4, which is on the lower right of the 555 timer.
Add a jumper from pin 4 (we used hole 17j) to the red (+) power bus (any hole along the red line at the bottom).
Polarity matters for electrolytic capacitors; they can safely go in only one way. Be careful which way you stick it into your breadboard. The positive (+) lead is longer. The usually shorter negative lead is marked with (–) signs on a lighter band.
Stretch the legs of your blue electrolytic capacitor and bend the ends at right angles so they can more easily go into the breadboard.
Connect the positive (+) lead to pin 3 (we used 18g) and the negative (–) lead to any hole in the top half, right side of the board (we used hole 9e).
All the other components you’ve used so far could be inserted either direction, but for the electrolytic capacitors, polarity matters.
< See blue note.
9 - Resistor (with green band)
The third band of the resistor multiplies the resistance value. This resistor has a value 100x greater than the orange-banded resistor you used in step 3.
To install your second resistor, find the 1MΩ one with bands colored | | | | brown, black, green, gold.
Use it to connect pin 2 (we used 19h) to the (–) ground bus (any hole along the blue line at the bottom).
10 - Jumper
Use a jumper to connect pin 2 to the row with your photoresistor and capacitor.
(We used holes 19g and 30h.)
11 - Jumper
Use another jumper to connect pin 1 (we used 20h) to the (–) ground bus (any hole along the blue line at the bottom).
12 - Jumper
Add a jumper to connect the top and bottom horizontal red (+) power buses on the breadboard, using any hole along each red line.
You don’t need to connect the blue (–) ground buses because all our ground connections are on the bottom bus.
13 - Speaker
A speaker is an output device, converting electrical current into sound. Tiny movements in the coil compress air to make sound waves. Sometimes speakers need to be connected with the correct polarity, but not in this circuit.
Time to give this circuit a voice! First, insert the red and black leads of the speaker into the green terminal block. To catch and lock the speaker leads, press the tiny buttons on top.
Notice that there are two prongs underneath the green terminal block. Insert the terminal so that one prong is in the same row where you inserted the negative (–) lead of the blue 100μF electrolytic capacitor. Make sure to keep some holes accessible in the next row over, where the other prong was inserted.
(We inserted our terminal block prongs into holes 8a and 9a.)
14 - Jumper
Add a jumper to connect any hole from that “next row” of the green terminal block (the one that’s not connected to the capacitor; for us holes 8c-e) to the (–) ground bus (any hole along the long blue line at the bottom).
15 - Battery Pack
Add batteries to your battery pack. Insert the black lead of the battery pack into the (–) ground bus (any hole along the long blue line at the bottom) and the red lead into the red (+) power bus (any hole along either red line).
Tip: use the wire strippers to remove a little insulation from your leads if you don’t get enough contact when the leads are fully inserted.
The back of the breadboard has a strip of double-sided tape if you’d like to secure your speaker and battery pack there.
Try It Out
That’s it! You should now hear a buzzing tone coming from the speaker. Move your fingers over the photoresistor to create different notes and sound effects.
The closer your fingers are to the photoresistor, the pitch should go down. To get a greater range of tones, try taking your “light theremin” from bright daylight to a dark corner indoors, or vice versa.
Replace your 10kΩ (Brown, Black, Orange, Gold) with other resistors to change the range of tones your speaker produces.
Circuit doesn’t make sound? Don’t despair!
Wiggle the leads on each component and make sure each one is inserted fully.
Go back through each step and double-check to see you have the correct connections. Some people even remove every component and start again.