8-Bit Software

The BBC and Master Computer Public Domain Library

Back to 8BS
Return To John Ilsley Articles

Light Pen Construction 07/10/2002  

Electronic Projects
Project 4a
Light Pens
By: John Ilsley (27N)

Added CJR: This article can be found on TBI-47. TBI-47 has software on it that the light pen can use. The Light sensor he lists is very difficult to obtain. I have a supply of a suitable replacement and can supply a kit to make this light pen.

Click the thumbnail for a larger image. This is the lightpen that I use as the 'reminder' when making them up myself using John's instructions. My soldering has improved greatly since I made this one by the way, I have had my cataracts whipped out.

Due to a number of 8BS members requesting information on light pens, I have decided to include details of how to build them in addition to the normal electronics projects. I must apologise to members who requested details and have had to wait for them. This was due to me being quite ill recently. However, I hope that this article will now help you to build your own light
pens and use the light pen menu in a previous 8BS issue.

Ok, now owing to the fact that this project uses no veroboard, it does make it hard for me to explain to the beginner, so if you do get stuck or don't understand something, then please don't attempt to mess around with the analogue port, get Chris or myself to build the light pen for you, and while false connections at the analogue port should not damage the computer, I promise, you will get quite a scare!

Please note, neither myself or Chris will accept any responsibility for any damage caused to yourself, the computer or any other item or parts.

Parts required
This is probably the most popular and cheapest version of the BBCs
light pen that there is. You require only a few items in all, these are:-

R1....... 1 10k resistor 5% (Brown, black, orange).
T1....... 1 2N3705 transistor
D1....... 1 TDET500C photosensor
W1 to 3.. 3 lengths of thin insulated wire of about one meter.
W4 and 5. 2 lengths of thin insulated wire of about six inches.
P1....... 1 15 pin 'D' type male analogue plug
B1....... 1 Old 'BIC' biro pen casing.
S1....... 1 Push to make miniature touch pad switch (optional).

Building the project
A light pen is very simple to build, however explaining it is not as easy, so please bear with me. Firstly, examine the transistor. Hold the transistor with the legs pointing down and the flat side towards you. The legs are:

On the left is the EMITTER. The middle leg is the COLLECTOR. The leg on the right is the BASE.

Strip and flux about two or three millimeters off the end of all of the wires at both ends.

Solder one end of W4 to one leg of D1. Solder one end of W5 to the other leg of D1. Solder W1 to one end of R1. Solder W2 to the COLLECTOR of T1. Solder W3 to the EMITTER of T1. The free ends of W1, W2 and W3 will eventually be soldered into the analogue plug.

Solder the BASE of T1 to the free end of R1. Now examine the photosensor D1 carefully. You will see a flat part on the outermost rim of the bulb. Solder the wire from the leg nearest to this flat part to the leg of R1 that is also soldered to the BASE of T1.

Solder the other wire from D1 to the EMITTER of T1. The miniature touch pad switch S1 may be inserted at a handy point in this wire

Here is a diagram that may help:

l                                           l  E=Emitter
l                            T1           l  C=Collector
l__D1__S1__E      C     B__l  B=Base
                       l       l      l         l and _ is wire
                       l       l     R1
                       l       l      l
                     W3   W2 W1

Insulate the bare bits of wire with insulation tape and gently insert the components into the biro until D1 sits about five millimeters from the narrow end of the pen. It would be advisable to put black tape around this end of the pen to stop unwanted light entering D1.

Position S1 on the outside of the biro close to the end where you will hold it, approximately 1 inch from the point. You will need to cut a slot in the biro casing and use glue to fix S1 in the correct position.

Examine the 15 pin 'D' type male analogue plug. It should be marked out with numbers : 1 to 15. If it isn't, then go onto next paragraph, if it is, then wire it as follows: W1 goes to pin 1. W2 goes to pin 9. W3 goes to pin 2.

If your 15 pin 'D' type male analogue plug doesn't have numbers, then hold the plug with the rear towards you and the longest side above at the top. W1 goes to first pin on the top right. W2 goes to the first pin on the bottom right. W3 goes to the pin on the top second in from the right.

Ok, well there is only one way to test this project. It doesn't matter if the computer is switched on when plugging in this light pen, so, first load up the file "LightPn" and run it. Insert the plug into the back of the BBC. If the computer crashes, switch it off and check the connections in the light pen and analogue plug.

If the computer is ok, hold the pen up to the screen and press the switch if fitted. The blob on the screen should move to the pen. If it doesn't, then the program needs changing. See text in the program for details. Now you're ok to use the light pen. Have fun!

If the computer is ok, but nothing happens, then first check all the connections ensuring that the analogue plug is wired up correctly. If nothing happens, then press 'Escape' and re-run the program. If still nothing happens, then open the BBC and see if you have the Analogue to Digital chip. If you haven't then see your dealer, if you have, then assume something is wrong with the transistor or the photosensor.

I hope that you can build this project, if however because these instructions are not too clear you find that you cannot, then please do not hesitate to contact me or Chris. It is a very intresting project to use, and can be a very useful aid. With work, the programs can be used to select, move, draw or anything, even turn the computer off... Yes, even that can be done with only a few components, a 250 volt 5 amp relay, a small program, and a small amount of wiring on a circuit board, cost of that would be about five pounds.

 Back to 8BS
Return To On Line Magazine