The Red Box
CONSTRUCTIONThe largest units in the red box are the telephone earpiece and battery. Since the unit works on 9V and has no coils, 1OV parts can be used. The push buttons are high quality units that won't bounce on you and fire the box twice. Grayhill makes some good ones. Snap action are the best, but they make noise when you hold the box up to the phone.
Different earpieces or speakers will produce different volumes. If a low impedance (8-32ohms) speaker is used, less volume and more battery drain will result. Increase the value of the 10 mfd. output capacitor with low impedance speakers, and increase 22K gain resistor if necessary.
Remember, with a red box you are going to play it for an operator who will easily recognise a faulty tone pulse. So make your solder connections tight & use good switches. Cover the speaker holes with a small patch of thin foam to prevent gook from flying Into the magnet.
OPERATING PRINCIPLESThe red box consists of four sections, the oscillator, the flip-flop, the timer and the amplifier. To understand how these produce the necessary tones we'll take for example a dime sound. A dime is two pulses of 2200 cycles, each being 60 milleseconds long, with a 60 ms. space between them. When you press the 10c. button, the flip-flop starts turning on and off, on for 60 ms. and off for 60 ms., continuously. This in turn shorts out the oscillator, producing on and off pulses to be fed to the amplifier and speaker. How do we get only 2 beeps? Well, when you press the button, the timer starts. After 180 ms. it turns off the flip-flop, thus stopping the tones. Other sounds are produced by changing the speed of the flip-flop and the delay of the timer. Also, the two RC pairs of 1M and .047 pulse the timer and the shorting transistor, to start the timer and flip-flop at the same time. Current drain is approx. 7 ma, so don't worry about the battery. You will hear some leakage after the tone sounds if you keep holding the button down, but this is alright if it's faint or if you let go.
TUNINGFirst, adjust the 30K oscillator trimmer to produce a beep when a button is pushed. Then, simply call a friend at a pay phone and have him deposit money. Tune the oscillator to the same pitch as the beeps you hear. If the oscillator is too high, raise the .0033 cap to .0047. If the range of the trimmer is too small, add a 30K series resistor.
Now, you should get one, two, and 5 beeps for 5, 10, and 25. If not, you'll need to adjust a bit. If you get only 4 beeps for 25, raise the 620K resistor. This may, however, change the 5 and 10 beeps. The 680K resistor is the number of beeps for the dime, and the 120K is for the 5 button.
The 150K resistor controls the speed of the flip-flop for 25. If you get 5 beeps,
but they're too slow compared to the phone your friend is at, you can speed
them up by lowering it. This will give you more beeps, however, in the same
period of time that used to give you 5 beeps. So then you should lower the 620K
until you get 5 beeps again, and then check for proper beeps on the other buttons
Always adjust 25 first, then do 5 and 10.
CALLINGGo to a pay phone, dial your number direct(after depositing a dime)(or 20c.) and when the operator comes on she'll ask for more money, and possibly even return your dime. Now, very slowly and quietly put the red box up to the mouthpiece and press one button per coin, and be sure to pause several seconds in between each coin, just as if you were reaching for the money in your pocket and fumbling to put in the coins. If the operator gets suspicious it could be for two reasons; first, you may be off tune or off speed.
In that case tune it. Second, the operator may be suspicious because in some areas a visual indication of insertion of coins is provided for the operator.. So the nperator may say something like "I'm sorry, sir; but that didn't register". Just mosey along to another phone booth. You'll find the red box is the safest box to use and easy and fun to use, too. Even the operator will thank you!
The Red Box