Votem Cassette Controller

If you have a VOTEM, or if this article talks you into buying one, here is a simple modification which will extend the board’s utility further, and allow you to start LOADing a program from tape, flip a switch when the loading bars appear on the screen, and walk away; when the program has loaded, the tape recorder motor is automatically shut off, and the tape remains there until you come back and flip the switch back on. If you don’t have a VOTEM, you can still use the information given about opto-couplers in your own interfacing projects, or add it to one of the circuits shown in “Cassette Connection.” But if you already have a VOTEM, you can get right on with it, or incorporate it into your unit as you’re building it up.

Most recorders have a REM jack for the remote control switch on “push to talk” or stand-by-switch microphones, This will go to the jack on VOTEM marked “audio out” (JACK 2) which will be re~defined (most applications have no need for this anyway, and if you do need it, use a “y” connector at the “tape signal out” JACK 3).

Total additional parts cost is about $4. The parts you’ll need are:

• 1 – mini-plug (goes to VOTEM)
• 1 – sub-mini plug (goes to cassette REM jack)
• 1 – optocoupler (high gain phototransistor type) (4N35, 4N36, 4N37, H11A1, TIL117, or eq.)
• 1 – medium power NPN transistor, e.g. R/S 276-2030)
• 1 – SPST miniature slide switch
• 1 – .01 uF. dise capacitor
• 1 – 330 K-ohm resistor, 1/8 or 1/4 watt.
• 1 – small silicon diode, 1N914 or similar Cable, hookup wire, epoxy or superglue.

Radio Shack sells an assortment of five couplers for $1.98 (276-1654) which will give you several to experiment with. There are basically two types of couplers; transistor (DC only) and triac (designed for switching AC) types. Transistor units are also available in a “Darlington” configuration (two transistors in cascade for higher gain) which require less LED drive current (higher overall gain). For our application, though, the ordinary transistor type is better as it has a lower on-state voltage drop than Darlingtons. The transistor Q1 should be capable of handling up to about 500 mA. and have a minimum gain of 50 or better. Our LED drive current will be about 7 mA., so with a coupler gain of .5 and transistor gain of 50 (worst case) we get a maximum output current of about 175 mA. Most recorders take under 100 mA. in “play” with the speaker disengaged, so we should be safe. If your circuit works but the motor runs slow or wows, try a higher gain transistor and/or coupler. (Couplers such as TIL111, MCT-2, 4N26-28 typically have a lower gain, .1-.2, so will require a higher gain transistor),

Here are the step-by-step modification instructions:

1. Remove capacitor C1 and jumper J4. Install C1 (.01 uF.) where J4 was.
2. Remove D2 (not really necessary and reduces accuracy. You’re better off without it. Down East Computers also recommends its removal to improve accuracy.)
3. Cut J1 in the center and bend the ends up. This will be our connection to the opto-coupler LED.
4. Cut the ground trace on both sides of JACK 2, If you wish, you can bridge the gap with a piece of insulated wire, but it’s not really necessary.
5. Cut the trace going to pin 10 of IC1, before the juncture to pin 8 and the DIP switch. (see diagram). Bridge the gap with a 1N914 or like silicon diode, banded end (cathode) end toward IC1 pin 10. Connect a .01 uF. capacitor and the 330K resistor from the diode cathode/ pin 10 to the adjacent ground trace.
6. Connect the emitter (pin 4) of the coupler to the base of the transistor, and the collector of the coupler (pin 5) to transistor collector. Connect the LED portion of the coupler (pins 1 and 2) to the ends of Jl, with pin 1 (anode) going to the one closer to the edge of the board. Don’t connect anything to pins 3 and 6.
7. 7 Cement the slide switch to the board between JACK 3 and JACK 5, with the handle protruding slightly so you can reach it with the covers on. Connect it directly across JACK 2. Be sure that the case of JACK 2 is indeed disconnected from VOTEM ground.
8. Connect the free end of Q1 (emitter) to JACK 2 negative (case) and the collector and OC pin 5 to JACK 2 positive (tip).
9. Make up the remote cable. The REM jack on the recorder is usually a sub-mini phone jack, so you’ll need a cable that goes from a sub-mini plug on one end, and a mini plug on the other, Make up the cable rather than buying a molded one, as you have to check polarity. Remove the outer cover from the sub-mini plug and insert into the REM jack on the recorder, place the recorder in PLAY. It shouldn’t run as the plug is inserted, but you should measure about 6V (or whatever battery voltage) across the exposed connections of the plug. Note the polarity: if the “tip” is positive, connect the cable as usual = tip to tip, case to case, But if the tip is negative with respect to the case on your machine, reverse the connections. The point is that the “tip” of the mini-plug going to VOTEM must be positive,

Hook up your VOTEM as usual for tape loading. Don’t hook up the remote cable just yet. Verify that the tape conditioner works by playing a program tape and entering LOAD “x”. The LED should glow steadily when program data is playing, and go out during silent parts or when tape recorder is stopped. Adjust volume so that the LED only flickers slightly during the pre- and post-save buzz. You can now tell exactly what’s going on with your tape signal.

If all is well so far, connect the remote cable. Flip the slide switch ON, and the recorder should work as if nothing had changed. LOAD an actual program tape, and when the actual program starts (LED on) flip the switch to “auto-off.” The motor should keep running. Motor current is now passing through the switch transistor, which is energized whenever LED is on by the opto-coupler. As long as the data keeps coming, the motor stays on. At the brief silence at the end of the program, the LED goes off and so does the motor. Also, if there is a drop-out, the motor will stop so you can inspect the tape at that spot. The time constant of the R-C filter we added is fast enough to catch even short lapses, while providing enough filtration to insure the LED driver comparator stays on during signal.

So you see how easy it is to use opto-couplers, and you have another handy feature on your computer system. Opto-coupler/transistor combinations like this can be used to switch any DC load. If you need more gain (output current) you can use Darlington couplers or extend the external darlington-connected transistors. The more transistors you cascade, though, the greater the on-state voltage drop will be; figure about .4 volts * (N+1) drop, where N is the total no. of transistors (including the one(s) in the coupler).

Coupling to AC circuits is just as easy, and triacs can be cascaded as far as you need with no change in on-state voltage, so you can control as big a load as you like. Example, a MOC3011 coupler can trigger a sensitive-gate 4 amp. triac like 2N6073 directly, or the 2N6073 can trigger a 10 amp MAC11-6 (Motorola part numbers). Circuit 2a shows the simplest approach, workable with resistive loads like lamps and heaters. For motors and other inductive loads (even equipment with large power transformers) you’ll need to use something like circuit 2b, which includes a “snubber” network to prevent the inductive “kick” from the load from falsely retriggering the triac. Exact values depend on the load; start with those shown and increase Cl as necessary if the triac gets stuck in the ON state.