Picture of Analog Beat Box
This is an analog circuit that plays a synthesized drum beat. It can be scaled in amount and the quality of the noises.  
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Step 1: What You Need

Picture of What You Need
1 power source
3 breadboards
Wires (Yellow, Blue, Red, and Other) if you feel like following conventions
2 Speakers
6 OPA551 Op-Amps
2 TL081 Op-Amps
4 2N3904 NPN Transistors
2 10kOhm potentiometers
2 diodes
2 LEDs (optional)
3 200kOhm
6 1kOhm
2 4.4kOhm
1 3.7kOhm
3 1.7kOhm
2 160kOhm
3 5.8kOhm
4 0.1uF
2 0.047uF

An Oscilloscope for debugging can come in quite useful.
Wire stripper/cutter

Step 2: The Circuit

Picture of The Circuit
It all goes together just like this. It's good to note that we only included a "bass" and a "snare". Prepare a circuit diagram of any other tones you wish to include in your circuit if you wish to add additional sounds on top of the basic ones we've included.

Step 3: Making the Beat

Picture of Making the Beat
This circuit uses a hysteretic oscillator as origin of the "beat". The hysteretic oscillator in this circuit has a frequency of about 1 hertz. The oscillator outputs a capacitor voltage "shark wave" which is compared using a comparator circuit to a steady voltage of about 2.5 volts. The 2.5 volts is made simply from a power source and a series of resistors. When the shark wave exceeds with the steady voltage, the comparator outputs 12 volts, essentially turning the circuit "on". When the shark wave is below the steady voltage, the comparator outputs 0 volts, essentially turning the circuit "off". For the second part of the beat, another comparator is used but compares the shark wave to -2.5 volts is used and that triggers in a similar fashion to the previous.

i'm really interested in experimenting with this, do you have any suggestions for a replacement for the opa551? thanks!

Since the outputs are buffered by the transistors, you don't need the
opa551. In fact, none of the outstanding features of the opa551 are
important for this (high voltage/"high" current out). A simple and
generic op amp like the lm124 (which has 4 op-amps in it) would probably

good to know, I'll start with whatever opamps i have and see how it goes. thanks!

rlychpmn (author)  choklitlove1 year ago

The opa551 was what we had readily available. I would agree that it was probably over specced for this application. The opa37 is quite a bit cheaper.

There are definitely some places in the circuit where a TL081 would have worked as well.

awesome, thank you very much!