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          A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

  Electronics 6

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The essential part of this timer is the SR flip-flop. It's function has already been explained here, although here is has the special feature of an inverted output. Thus, if the set-input is 'high', then the output is 'low'. Resetting causes it to return to high.

The input to an amplifier is then also inverted on it's way further to output 3. Through having the basis of a transistor, which is probably the cause of the double inverting, the possibility has been created, of connecting output 7 with earth.

Indeed, that which interests us, are the two operational amplifiers on the left. Their effect is used as a voltage comparator. If the voltage on the plus-input is higher than that on the minus-input, the output goes onto high. This is valid for both operational amplifiers. However, the upper one switches the set- and the lower one, the reset-input.

On the left, between connection 8 and earth, voltage supply prevails, which is distributed by the serially switched resistors. If we were to assume 12 volts, a higher partial voltage would thus be found over the center resistor and, a lower, below the center resistor. Below the lower partial voltage, the output goes onto high. This condition is maintained, if the partial voltage is exceeded.

This by the way, is exactly what happens with the condenser on the right, it is charged. If now the upper partial voltage is also surpassed, the output goes onto low. A flashing light on 3 would go out. The problem is: The condenser must slowly lose it's charge and that happens through the output 7, which is now, through the transistor, connected with earth.

As you've probably guessed, as soon as the voltage on the condenser drops below the lower partial voltage, the flashing light is switched on again through the lower operational amplifier. Another special feature of this circuit: The charging- and discharging times are, through the two serially switched resistors on the right, uneven, that means, the switch-on time is greater than the switch-off time. 10/13               Top of page               Index
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Translator: Don Leslie - Email:

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