Parallel Port Transistor Based Interface

Transistor Interface

I got most of the basic stuff ironed out with the interface.  It turns out that PNP transistors are in fact needed for triggering Positive voltage operated buttons. No biggie just have to make some noteson the schematic about which in to use when, otherwise no modifications are needed. I also got the software compiled and it can be downloaded here.

Pattern Sequencer Software Version 0.1

An old computer is suggested to run this ( 200 Mhz or less), preferably one that you wouldn’t mind frying if something went sour, although I’ve been using my old laptop for weeks while experimenting and its just fine. The entire program can be ran off a basic DOS bootup floppy. In the future a fully loaded Disk image will be provided. Comments and suggestions are welcome.
*End Update*

At one point or another I’m sure everyone thought about triggering sounds of circuit bent toys in a patten like fashion with the use of a computer. This is my version of a super basic interface, which operates on the fact that transistors make great switches and will work perfectly for triggering sounds of circuit bent toys or DIY Sound circuits, being the equivalent to pushing button. When coupled with a pattern based software like this one from to turn on and off the digital pins of the computer’s parallel port any toy circuit can suddenly become a whacked out drum machine. This interface works by having a digital pin from the parallel port or any other controller board connected to the Base of the PN2222 NPN transistor. The Collector of the transistor is connected to the Ground of the Toy and the Emitter is connected to the button trace of a Toy which triggers a sound. When the Base goes to HIGH the Emitter and the Collector become bridged. Subsequently when the digital pin goes back to LOW on the computer’s parallel interface the bridge gets broken and no sounds will be triggered for the time being. Some toys require the connection to be sustained for a few milleseconds, so the computer program must accommodate that with delay routines. Most of the toys that I have looked at use a Ground signal to trigger their functions. However there are exceptions when a positive voltage is needed for this case a PNP transistor must be used instead of an NPN.

The computer parallel port has 8 digital output pins that can trigger up to 8 sounds individually, however in my schematic above I only have 4 connections, this is done to simplify the diagram.

The above circuit will be more less my topic for the Bent Festival Workshop along with a few other tips and tricks to get a bent instrument interfaced to a PC. The plan is to have a DOS based self booting diskette with a graphical step sequencer to utilize this interface. So far I have written a program that will do basically the same thing in Processing, except that Processing doesn’t come with a set of commands to control the parallel port, so I’m sending serial commands to an Arduino Interface and turning off and on its digital pins to accomplish virtually the same thing.

The diagram presented is a very rough draft of the final circuit, I could have made a huge mistake detailing this, so if you try building it and it doesn’t work for you, please let me know. Maybe my Es and Cs are crossed or something. Good luck and more updates will come soon.

*Update* It was pointed out in the comments that connecting the Collector to the Toy Button trace and the Emitter to the Ground is the correct way of doing this. I tested that configuration and in fact it does also work. The voltage drops were more-less close with the transistor being connected either way. However I am not sure why this happens, but just to be a conformist I will rewrite the information to reflect that.

Also, I have noticed that one of my keyboards is actually triggered by having positive +5 volts connected to the button traces. The above interface did not work correctly for that with an NPN transistor, however it did sort of work with a PNP one. I need to still do more work researching that.

Here is a list of what I tested so far.
Ground Triggered Toys:

Hing-Hon Square wave keyboard.-Works

Playskool drums.-Works

Positive Triggered Toys:

Kawasaki DSI-like keyboard ( Black Blob with LED keys ) – Does not work yet.