Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Simple 6-input alarm circuit

This simple alarm circuit was designed for use in a combined garage and rumpus room. It can be assembled on Veroboard and uses just one IC plus a handful of cheap components.

The circuit is based on a straightforward 555 timer circuit (IC1). This is wired as a monostable and sets the siren period which is adjustable up to about three minutes using potentiometer VR1.

In operation, IC1's pin 2 input monitors the detector circuit for negative-going signals. When a switch is closed, a brief negative-going pulse is applied to pin 2 via a 10μF capacitor and its corresponding series diode (D2-D7). This triggers IC1 which switches its pin 3 output high and switches off relay RLY1 (ie, RLY1 is normally on).

As a result, the piezo siren sounds for the duration of the monostable period. In addition, relay RLY2 is turned on via diode D9 and latches on via D10. This means that the strobe light (which is wired to the normally open contact) will continue to flash until the alarm is switched off (via the keyswitch).

At the end of the monostable period, RLY1 turns off and this turns off the piezo siren. The circuit can then be retriggered by any further trigger inputs from the switches.

A variety of detectors with normally open contacts can be used for the switches, including reed switches, pressure mats, IR detectors and glass breakage detectors. All switches must be open before the alarm is switched on.

R. Love,
Highbury SA. ($40)

Courtesy light extender

In essence, this circuit is a 15 to 20-second courtesy light extender for cars. It is activated in the usual way by opening a door but it also samples the negative lock/unlock signals from a car alarm or central locking and does two more things.

First, when an unlock signal is received, it turns on the courtesy light for 15-20 seconds before you open the door. Second, when a lock signal is received, it turns off the courtesy light immediately, with no fade-out. This is done to eliminate false triggering of the burglar alarm through current drain sensing.

When a car door is open or the unlock relay is activated, the 33μF capacitor discharges through diode D1 and this keeps transistor Q1 turned off. This allows Q2 and Q3 to turn on and the courtesy lamp is activated.

When the door is closed, the courtesy lamps stay illuminated and the 33μF electrolytic capacitor starts charging through the associated 1MΩ resistor. As the voltages rises, Q1 turns on slowly, turning off Q2 and Q3 which gradually fades out the courtesy lamp.

If a lock signal from the central locking system is received, relay 1 closes and charges the capacitor instantly, so the lamp turns off immediately.

Relays were used to interface to the central locking/alarm system as a safety feature, to provide isolation in case something goes wrong.

Matt Downey,
Marleston, SA. ($35)

Battery replacement power supply

Your child's battery toy has failed and you have to fix it. Once you have managed to get it apart, the battery compartment is not likely to be connected to the works or the batteries might have gone flat anyway. The solution is this switchable supply which is designed to replace from one to six dry cells. It is not intended to replace the batteries on a permanent basis, as in most cases this is not practical.

The heart of the supply is an LM317T adjustable 3-terminal regulator and six trimpots selected by switch S1b. The other pole of the switch, S1a, is used to select taps on the transformer secondary, to minimise power dissipation in the LM317T. The table shows the trimpot settings for the six voltage outputs.

Diode D1 and the 10μF capacitor and the LED provide power indication. This has the advantage of constant brightness which would not be obtained if the LED was run from the unregulated switchable DC.

Philip Button,
West Moonah, Tas. ($35)

Automatic headlight reminder

Do you drive an older car without an automatic "lights-on" warning circuit? If so, you've probably accidentally left the lights on and flattened the battery on one or more occasions.

This headlights reminder circuit will prevent that. It's more complicated than other circuits but it's also more versatile.

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As shown, the circuit uses two low-cost ICs. IC1 is a 555 timer which is wired to operate in astable mode. Its output clocks IC2, a 4017B decade counter. IC2 in turn drives a row of indicator LEDs and also resets IC1 (after about 10s) via transistor Q2.

The circuit works like this: when the ignition is on, transistor Q1 is also on and this pulls pin 4 of IC1 low. As a result, IC1 is held reset and no clock pulses are fed to IC2.

Conversely, if the ignition is turned off, Q1 will turn off and so IC1 will start oscillating and sound the piezo siren. At the same time, IC1 will clock IC2 and so LEDs 1-10 will light in sequence and stop (after about 10s) with the last LED (LED10) remaining on. That's because, when IC2's O9 output (ie, pin 11) goes high, Q2 also turns on and this pulls pin 4 of IC1 low, thus stopping the oscillator (and the siren).

Note that different coloured LEDs are used to make the display look eye-catching but you make all LEDs the same colour if you wish. Installing optional diode D1 will alter IC1's frequency and this will alter the display rate.

Finally, if the lights are turned off and then back on again, the alarm will automatically retrigger. LED1 is always on if the lights are turned on. If you don't want the LED display, just leave the LEDs out.

L. Marshall,
Barrack Point, NSW. ($40)

Speed alarm for cars

In normal suburban driving you pass through so many different speed zones that it can be a nuisance having to switch speed settings. The speed display can also be a distraction. This circuit eliminates the display and the need for speed selection. Each time you exceed a particular speed setting (eg, 40km/h, 50km/h, etc), a piezo buzzer will beep.

Speed pulses are fed to the base of Q1 and the resulting waveform at its collector is fed via an RC network to the input of an LM2917 frequency-to-voltage converter, IC1. The resulting voltage is fed to three comparators (IC2d-IC2b) which have the reference voltages at their inverting inputs set by 10-turn trimpots VR1, VR2 & VR3.

The output of each comparator is applied via another RC network to the gate of an SCR. The anodes of the three SCRs are commoned connected to the inverting input of the remaining comparator, IC2a. Its non-inverting input is set to +2.3V by trimpot VR4.

In use, once you exceed the speed setting for a particular comparator, its associated SCR briefly conducts to pull pin 2 of IC2a low and a short beep is emitted by the piezo buzzer. Then, as you exceed the next speed setting, another beep will be heard.

The idea is make each speed setting a few km/h higher than actual so that if you are driving at the correct speed in a given zone, the buzzer will not sound. But as you increase speed, the buzzer will beep once as you exceed the speed setting for each zone.

In this way, there is no need to continually switch speed settings as you drive through different zones and you can choose to ignore beeps that are not "illegal".

Col Edwards,
Rosslea, Qld.

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