# Loop Sensor

Many security systems use a closed loop of wires and switches arranged so that whenever a door or window is opened, the loop will be broken and the alarm will sound. An obvious problem is that someone can tamper with the system, short out the loop, and later on, come back and burglarize the premises without sounding the alarm. Hiding a known resistance in the loop can prevent this. That way, the alarm can distinguish a short circuit from a correctly functioning closed loop.

The figure above 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 the 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. The figure below shows the states of outputs A, B, and C under different loop-resistance conditions. 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.

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.

# Water Activated Alarm

The circuit uses a 555 timer wired as an astable oscillator and powered by the emitter current of the BC109C. Under dry conditions, the transistor will have no bias current and be fully off. However as the probes get wet the transistor will conduct and sounding the alarm.

An On/Off switch is provided and remember to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of veroboard. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).Parts:1x BC109C
1x NE555 Timer
Misc transistors, condensators, resistors, etc.

This simple circuit is sure to have the police beating a path to your door – however, it has the added advantage of alerting you to their presence even before their footsteps fall on the doormat.

The circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b. When a hand or a foot approaches the sensor, the frequency of the transmitter (IC1a) drops appreciably.

Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is tuned to a frequency just below this. The 1MHz transmission will therefore not be heard by the radio. But bring a hand or a foot near to the sensor, and the transmitter’s frequency will drop, and a beep will be heard from the radio.

Attach the antenna to a multiplug adapter that is plugged into the mains, and you will find that the Medium Wave transmission radiates from every wire in your house. Now place a suitably tuned Medium Wave radio near some wires or a plug point in your house, and an early-warning system is set up.

Instead of using the sheet of tin foil as the sensor, you could use a doorknob, or burglar bars. Or you could use a pushbutton and series resistor (wired in series with the 33K resistor – the pushbutton would short it out) to decrease the frequency of IC1a, so activating the system by means of a pushbutton switch. In this case, the radio would be tuned to a frequency just below that of the transmitter.

# Gate Alarm

A cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply.

IC1a is a fast oscillator, and IC1b a slow oscillator, which are combined through IC1c to emit a high pip-pip-pip warning sound when a gate (or window, etc.) is opened. The circuit is intended not so much to sound like a siren or warning device, but rather to give the impression: "You have been noticed." R1 and D1 may be omitted, and the value of R2 perhaps reduced, to make the Gate Alarm sound more like a warning device. VR1 adjusts the frequency of the sound emitted.

IC1d is a timer which causes the Gate Alarm to emit some 20 to 30 further pips after the gate has been closed again, before it falls silent, as if to say: "I’m more clever than a simple on-off device." Piezo disk S1 may be replaced with a LED if desired, the LED being wired in series with a 1K resistor.

Figure 2 shows how an ordinary reed switch may be converted to close (a "normally closed" switch) when the gate is opened. A continuity tester makes the work easy. Note that many reed switches are delicate, and therefore wires which are soldered to the reed switch should not be flexed at all near the switch. Other types of switches, such as microswitches, may also be used.

R1 = 10K Ohm
R2 = 38K Ohm
R3 = 100K Ohm
VR = 25K Ohm potmeter
C1 = 10 uF
C2 = 10 nF
C3 = 10 uF
C4 = 100 uF
D1 = 1N4148
S1 = Piezo disk (or LED with 1K Ohm resistor ini series)

The relay is energized by entering the first four digits of your chosen five-digit security code. Entering the full five-digit security code – will de-energize the relay. When “A, B, C & D” are pressed in the right order – and within the time set by C1 and R2 – current through R11 switches Q6 on. This energizes the relay. When “A, B, C, D, & E” are pressed in the right order – and within the time set by C1 and R2 – current through R7 switches Q5 on. This de-energizes the relay. The circuit was designed to control the Modular Burglar Alarm System – but it will have other applications. A simpler – A Four-Digit Version – of the circuit is also available.

Choose the five-digit security code you want to use – and connect the keys to “A B C D & E”. Wire the common to R1 and all the remaining keys to “F”.

The circuit is easy to use. When you enter the first four digits of your security code – the relay energizes. The 12-volt output moves from the “off” to the “set” terminal – and the green LED lights. At the same time – R12 takes over the job of supplying base current to Q6. In effect – the relay latches itself on.

To de-energize the relay – you need to press keys “A B C D & E” in the right order. When you do so – pin 10 of the IC goes high – and it switches Q5 on through R7. Q5 connects the base of Q6 to ground. This switches Q6 off – and the relay drops out. The 12-volt output moves from the “set” to the “off” terminal – and the green LED is extinguished.

Any keys not wired to “A B C D & E” are connected to the base of Q4 by R9. Whenever one of these “Wrong” keys is pressed – Q4 takes pin 1 low. This discharges C1 – and the code entry process fails. If “C”, “D” or “E” is pressed out of sequence – Q1, Q2 or Q3 will also take pin 1 low – with the same result. If you make a mistake while entering the code – simply start again.

[b:7f913a99fe]Notes[/b:7f913a99fe]
The Keypad must be the kind with one common terminal – and a separate connection for each key. On a 12-key pad – look for 13 terminals. The matrix type with 7 or 8 terminals will NOT do. With a 12-key pad – almost 100 000 different codes are available. If you need a more secure code – you could simply use a bigger keypad with more “Wrong” keys wired to “F”. A 16-key pad gives over half a million different codes.

The Support Material ([url]http://uk.geocities.com/ronj_1217/5_digi.html[/url]) for this circuit includes a step-by-step guide to the construction of the circuit board, a parts list, a detailed circuit description and more.

For PCB layout see k5l.png

Look at [url]http://uk.geocities.com/ronj_1217/al1/fourd.html[/url] for a detailed description.
Ron J’s Circuit Page: [url]http://uk.geocities.com/ronj_1217/circ.html[/url] – updated regularly.
Write To Ron: [url]http://uk.geocities.com/ronj_1217/mail.html[/url]

Pressing a single key on the keypad – will energize the relay. Entering a four-digit code of your choice – will de-energize it. The circuit was designed to control the Modular Burglar Alarm System – but it will have other applications. If you require added security – A Five-Digit Version – of the circuit is also available.

[b:bc1035d987]Notes[/b:bc1035d987]
The relay is energized by pressing a single key. Choose the key you want to use – and connect it to terminal “E”. Choose the four keys you want to use to de-energize the relay – and connect them to “A B C & D”. Wire the common to R1 and all the remaining keys to “F”.

The circuit is easy to use. When you press “E” – current through D2 & R9 switches Q5 on – and the transistor energizes the relay. The 12-volt output moves from the “off” to the “set” terminal – and the green LED lights. At the same time – R10 takes over the job of supplying base current to Q5. This means that – when you release “E” – the relay will remain energized.

To de-energize the relay – you need to press keys “A B C & D” in the right order. When you do so – pin 10 of the IC goes high – and it switches Q4 on through R8. Q4 connects the base of Q5 to ground. This switches Q5 off – and the relay drops out. The 12-volt output moves from the “set” to the “off” terminal – and the green LED is extinguished.

Any keys not wired to “A B C D & E” are connected to the base of Q3 by R7. Whenever one of these “Wrong” keys is pressed – Q3 takes pin 1 low – and the code entry process fails. If “C” or “D” is pressed out of sequence – Q1 or Q2 will also take pin 1 low – with the same result. If you make a mistake while entering the code – simply start again.

The Keypad must be the kind with one common terminal – and a separate connection for each key. On a 12-key pad – look for 13 terminals. The matrix type with 7 or 8 terminals will NOT do. With a 12-key pad – over 10 000 different codes are available. If you need a more secure code – you could simply use a bigger keypad with more “Wrong” keys wired to “F”. A 16-key pad gives over 40 000 different codes.

The Support Material ([url]http://uk.geocities.com/ronj_1217/4_digi.html[/url])for this circuit includes a step-by-step guide to the construction of the circuit board, a parts list, a detailed circuit description and more.

For PCB layout see k4l.png