MINI ELECTRONIC LAPTOP -COJON-DRUM BOX

This is a small amplified instrument with which you can easily add percussion to any sing song or impromptu gathering of musicians. It fits easily on a one’s lap and played through a guitar amp has surprising tonal quality. I learned what not to do by building a prototype which can be found here: https://www.instructables.com/ELECTRONIC-LAPTOP-CAJON/

Some of the errors I made with the first one:

1. Cardboard does not have resonance, no matter where it is hit, the sounds are basically the same. Wood is a better choice. 2. Sensors on the top (striking surface) are too close and give a too-sharp sound. All sensors should be on the bottom. 3. Solid wire breaks with vibration- use stranded. 4. Don’t solder directly to the sensors for the same reason- the wire detaches easily. 5. A volume control is essential, especially if accompanying a guitar player using the same amplifier.

 I built this second iteration hoping to solve all those problems, using Baltic birch plywood which is solid, has good resonance, and has no interior voids.

Hardware items: 5mm Baltic birch, and 3mm Baltic birch- This is available from good hobby stores as it is used widely in model building. White glue. 5 minute epoxy. A scrap piece of 1/2” MDF. Wood screws.

Electronic items: Some fine stranded hook- up wire, a 6 foot connector cord with 1/8” plugs, (the kind used to connect your phone to other audio devices. (This is a stereo cord but connected to both monaural jacks the 2 stereo channels act as one), a 1/8” to 1/4” audio plug adapter, and a 1/8” monaural female socket. (alternatively, a 1/4” monaural socket and a guitar connector cord could be used) A 500K audio potentiometer and, most importantly, 4 piezo transducers. These are available from electronic suppliers. I ordered mine online from: digikey.ca. 

 It’s the part that makes those beeping sounds from your coffee maker or microwave. For that, however, it requires an oscillator circuit. It works just as well as a mini speaker or microphone, and in this case functions like a guitar pickup to detect drumming sounds. 

Part#- 7BB-20-6L0 Price: $1.18 CDN -cheaper if you buy 10 or more.

You will need solder and a soldering iron, drill and drill bits. Wood working tools for the box.

Build the box. Mine is 12” L, 6” W AND 5” D. Use 5mm for the sides and 3mm for the top. Take your time and use glue only. Simple lap joinery is the strongest and best. Cut a piece of the MDF to fit snugly inside the box as a bottom. This gives it weight and is a stable surface for the electronics. Play the box like a drum to see if it has proper resonance. Mine did, but you may have to glue some sticks of wood to the inside top to get the right sound. Also, drilling a hole in each side will greatly add to the sound quality. Also drill small holes near the bottom to attach the MDF base. To be whimsical, I made the box look like an upside down crate, and sprayed it with 3 coats of clear lacquer to make it more durable. A diagram is provided.

Prepare the base by drilling 4 shallow holes sightly larger than the transducers, using a Forstner bit. I held the transducers in place with 5 minute epoxy. Also drilled holes for the potentiometer and socket. Turn it over and drill a larger hole from the bottom over the mounting hole for the potentiometer. This will allow access to it and, since it is recessed, allow the box to sit flat when not in use. I attached 2 strips of wire above and below the transducers as a +ve and -ve buss bar to make connections easier.

A wiring schematic is provided but it is imperative to connect the transducers in parallel, that is, red to red, and black to black. Solder the leads directly to the +ve and a -ve buss bar on each side of the transducers. Red side should connect to one side of the potentiometer and the center connector of the potentiometer to the tip connector of the socket. Ground the other, 3rd connector to -ve ground. Black side connects to the sleeve (outside/ ground) connector of the socket.

Before attaching the box, plug in the cord, connect to your amplifier and do a test. Tap each transducer and you should hear a tap sound. Try the volume control. If it works in reverse, change the outside potentiometer connections.

If it all works, carefully attach the box with wood screws and start drumming. Experiment with the volume. You’ll be surprised at the effects, slapping various places on the top, as well as the front and sides gives varying tonal qualities. If accompanying someone on a guitar using the same amp, I suggest using a mixer.