FIG.l – ONE OF THREE OUTPUTS goes low depending on whether loop resistance is too high, too low, or just right.

Figure 1 shows a circuit that does the job. It”s a somewhat unusual application of a National Semiconductor LM3915 IC, normally used to drive LED” bargraph displays. That chip happens to contain the right combination of comparators and logic circuits to do what you need.

Step 1 is to translate the loop resistance into a voltage; that”s done by putting it into a voltage divider with resistors R1 and R2. Capacitor C2 protects the circuit against electromagnetic noise-important because burglar alarms use long wires, often running near heavy electrical equipment.

Step 2 is to translate the voltage into a logic signal indicating whether it”s in resisthe correct range. That”s where the LM3915 comes in. Normally, the LM3 9 15 would drive ten LEDs, one for each of ten small ranges of voltage. To obtain logic-level outputs, we have it driving 1K resistors instead of LEDs. Since we only need to distinguish three situations, not ten, we tie some of the outputs together. The LM3915 has open-collector outputs that can be paralleled in that way.

FIG.2 – THIS TRUTH TABLE shows the states of outputs A, B, and C under different loop-resistance conditions.

The truth table in Fig. 2 shows how the outputs work. Note that they use negative logic (OV for “yes”, +5V for “no”), the opposite of ordinary logic circuits. You can use inverters such as the 74HC04 to produce positive logic signals if that”s what you need.

Finally, note that the circuit will actually work with any supply voltage from 3 to 25 volts. Of course, if the supply isn”t 5 volts, the outputs will not be compatible with j-volt logic circuits.

This is the circuit diagram of sound effects generator. All sounds in this circuit are generated by HT2884. The HT2884 is a CMOS LSI chip equipped with tone circuit, noise circuit, and other control logic to generate various sounds to be used in sound effects production. This IC features: single power supply (2.4V ? 3.3V), low stand current, auto power-off function, and eight different sound sections. There are 8 different sound effect can be produced that are 2 lazer guns, 1 dual tone horn sound, 2 bomb sounds, 2 machine gun sounds and a rifle shot sound.

Circuit Notes:
This circuit works with a 3 Volt battery or 2 pieces 1.5V battery wired in series, but the IC will work with any voltage between 2.5 and 5 Volts. Switch S1 is the on / off switch. The output at pin 10 is amplified and drives a small 8 ohm loudspeaker. Pressing S3 once will generate all the sounds, one after another. S2 can be used to produce a single sound effect, next depression gives the next sound effect. Standby current is about 1 uA at 3 Volt, so battery life is very economical.

This is Electronic Siren circuit diagram which use standard discrete components. The sound produced imitates the rise and fall of an American police siren. When first switched on the 10u capacitors is discharged and both transistors are off. When the push button switch is pressed to 10u capacitor will charge via the 22k resistor. This voltage is applied to the base of the BC108B which will turn on slowly. When the switch is released the capacitor will discharge via the 100k and 47k base resistors and the transistor will slowly turn off. The change in voltage alters the frequency of the siren.

The siren sound generated from the oscillation process. Oscillator action is as follows:
As the BC108B transistor switches on its collector voltage falls and so the 2N3702 transistor is switched on. The process is very fast ( less than 1us). The 22n capacitor will charge very fast as well. As this capacitor is wired between the collector of the 2N3702 and the base of the BC108B, it soon reaches almost full supply voltage. The charging current for the capacitor is then much reduced and the collector emitter voltage of the 2N3072 is therefore increased; the collector potential will fall. This change in voltage is passed through the 22n capacitor to the base of the BC108B causing it to come out of saturation slightly. As this happens its collector voltage will rise and turn off the 2N3072 transistor more. This continues until both transistors are off. The 22n capacitor will then discharge via the 100k, 22k resistor, the closed push button switch, 9V battery, the speaker and 56 ohm resistor. The discharge time takes around 5-6msec. As soon as the 22n capacitor is discharged, the BC108B transistor will switch on again and the cycle repeats. The difference in voltage at the collector of the BC108B (caused by the charging 10u capacitor) causes the tone of the siren to change. As the 10u capacitor is charged, the tone of the siren will rise, and as it is discharged, it will fall. A 64 ohm loudspeaker may be used in place of the 8 ohm and 56 resistor, and the values of the electronic components may be changed to generate different sound effects of the siren.

Current drain is fairly high in this circuit so a suitable power supply is needed. The duration the tone takes to rise and fall is determined by the 10u and 22k resistor. These values may be changed for different effects.