Here are a couple articles that introduce you to control applications for the T/S computers (ZX81, TS1000, TS1500) with a minimum of effort and (best of all) at reasonable cost.
Before we control something with a computer, we usually need some way of getting information back to the computer; ie, to control an oven, the computer has to know the present temperature. Getting analog information (smoothly continuous like in the “real world”) into digital form (on or off, nothing in between) is called, aptly enough, “analog to digital conversion” or more simply A-D (read “A to D”). Several approaches are possible, but as usual there are trade-offs. The faster and more accurate you want the data, the more hardware it will take (and the more expensive it gets). A very sophisticated data acquisition system is available from Eric Reiter’s Computer Continuum (301 16th Ave., San Francisco, CA 94118) for about $200; software is also available to literally turn your ZX/TS into a multi-function storage oscilloscope, chart recorder, spectrum analyzer etc. The board can be adapted to other machines as TS2068, Apple, etc. This is the way to go if you need that kind of performance; and the price is considerably less than similar add-ons for other machines.
But wait! There is another way to go. For $42.95 incl. shipping you can get a VOTEM kit (Down East Computers, PO Box 3096, Greenville, NC 27834) and get accurate single-channel data acquisition. (Just a little later, though, we’ll show you how to get several channels.) VOTEM uses a conceptually simple approach; convert the analog voltage to a frequency, then use the computer to count how many times the signal changes state during a loop of preset length. So all it really takes is a V-F converter of some sort and a short machine code routine to read the EAR jack and keep track of the counts. This simplicity makes this approach easy to use, plus you can trade speed and accuracy as required for your application.
VOTEM is more than this, though; since it’s easy to understand and comes with extras like a tape-signal conditioner, temperature probe and a friendly, very complete text, it is a powerful educational tool. Only high quality components are used, the V-F converter itself is very accurate and stable. You can get the manual separately if you’d rather build your own from the ground up, but it’s hard to beat Down East’s price, all things considered.
So now you have a way of getting an analog voltage into your computer memory. From there you can print it, plot it, manipulate it any way you want. But how about going the other way? How can you get your computer to control other devices in response to software command (such as turning on a heater when temperature drops below a certain point?) Again, there is a relatively cheap but. effective add-on available; the BB-1 control module from Byte Back Co. (generally referred to as a “Byte Back Module” although the company has other products, including memory packs with attractive specs and price tags). This allows you to control eight devices with a simple POKE. It includes on-board relays for isolation and (relatively) high power switching. Tom and I both have one and find it an excellent product; more about this module elsewhere in this issue.
Meanwhile, there’s another option available to you. After receiving a letter from Chris Bannister, in which he described his “Blue Box” (built by Paul Hunter) which places the four 2K blocks of Hunter board memory under software control, I wrote to Dr. Hunter to ask if he’d be interested in sharing the details. He promptly responded with the beautifully written article you will find in this issue. If you have a Hunter board (if you don’t, you should), then you can use the output of the circuit to select 2K blocks and duplicate the infamous Blue Box, or you ean control whatever your heart and application desires. How about selecting one of four inputs to your VOTEM? (Aha! I can hear those wheels turning now.)
The final step in interfacing your computer to the outside world is providing the necessary isolation and power amplification required to operate “real world” devices as motors, heaters, lights, etc. Most of you are familiar with relays, which use an electromagnet to close a mechanical switch. The switch contacts are electrically isolated from the coil, and handle much greater current than is required by the coil; it is thus both an isolator and amplifier. But even the relay coil takes far more current than can be provided by the usual digital IC’s; additional amplification is required in the form of “relay driver” circuits. An improvement is the “solid-state relay.” The heart of such devices is the “opto-coupler.” An opto-coupler consists of an LED and a light-sensitive transistor in the same package but not electrically connected. The transistor controls, in turn, a larger transistor, which is the actual load switch. This approach is much cheaper than mechanical relays, and in, many ways more reliable. For moderate loads (up to a few amps) each SSR costs only about $5 max.
As a prelude to Paul Hunter’s article, I present a modification to VOTEM which adds yet another feature to this potent little “out-board”.