This home security project is based on a PICAXE-08 which monitors two infrared light beams, at the top and bottom of a flight of stairs. At the bottom of the stairs, an IR transmitter diode is mounted in the stair column and an IR receiver (sensor A) is installed in its opposite number. Therefore the 38kHz modulated IR beam is broken when a person enters the flight of stairs. A similar arrangement is installed at the top of the stairs (sensor B).

An LDR (light dependent resistor) determines whether it’s day or night (ADC numbers) and the program then changes the action of both IR sensors. When a person comes up the stairs during the day, sensor A triggers two wireless remote door bells, one in the house and one in the back shed via a relay, while sensor B is inactive.

A person going down the stairs in daylight triggers sensor B to put a 20-second hold on sensor A so that the bells don’t ring. At night, a person going up the stairs triggers sensor A to cause the bells to sound as well as switching on two floodlights to illuminate the front of the house for three minutes. When the B sensor beam is broken by a person going down the stairs at night, the floodlights come on again for three minutes and the doorbell is again inhibited for 20 seconds.

There are two other optional input devices: a pushbutton switch inside the house and an LDR circuit that detects car headlights coming up the driveway. The pushbutton will not ring the bell unless the headlight monitor is triggered. Both turn on the floodlights. The circuit works as follows:

IC1 is a 555 timer configured as an astable oscillator operating at around 38kHz to drive both IR diodes via 150Ω resistors. When either sensor A's or B’s beam is broken, its respective IR receiver (IRRX1,2) goes high. The respective PICAXE input pin 3 or 4 is also pulled high via diodes D1 or D2.

These input pins can also be pulled high by pushbutton switch S1 or the headlight sensor, via their respective diodes (D3 or D4) and 4.7kΩ resistors. In fact, D1 & D3 and D2 & D4 and their respective 4.7kΩ resistors comprise OR gates to control the PICAXE inputs.

The program determines the output state of PICAXE outputs 2 and 0. When driven high, output 2 switches the solid-state relay on via transistor Q1.

When output 0 goes high, it switches RELAY1 via transistor Q2 to drive the door bells.

The LDR type and series resistor used in the headlight sensor are a matter of choice, as outlined in the PICAXE article in the March 2003 issue of SILICON CHIP. It depends on where the control box is installed, as does the ADC numbers for the determination of day and night.

Another important point to remember is if you are programming your PICAXE on a separate board to your project, you may need to tie the serial pin input to ground via a 27kΩ resistor (or thereabouts) on the project board.

PICAXE-based car speed alarm This circuit monitors the speed of a vehicle using a Hall Effect sensor and magnets attached to a drive-shaft or half shaft, (as described in the Digital Speed Alarm for Cars, November & December 1999). When the magnets go past the Hall sensor, an input signal is fed to IC1 which is a TL071 op amp connected as a comparator. Trimpot VR1 sets the threshold and the output signal is a square wave fed to one half of a 4013 D-type flipflop, IC2. The output of IC2 equates to half a revolution of the wheel, assuming the Hall Sensor is monitoring a vehicle’s half shaft. The PICAXE-08 (IC3) measures the pulse width in milliseconds. It does this four times which equates to four revolutions of the wheel. IC3 stores that information in a memory location. This stored value has a direct relation to the speed of the car. After that, it just keeps comparing the next four readings with the first value stored. If the value is higher, the car is going slower. If lower, the car is going faster. Therefore, it turns on the green or red LED. It also calculates the 5% over-speed value which turns on the piezo buzzer. VR2, the LDR and transistor Q1 control the brightness of the LEDs, to suit the ambient light. To set the speed, turn the unit on (via S1) when the vehicle is doing the desired speed. For a different speed, switch the unit off and then back on again when the desired speed is reached. Power for the circuit comes from the vehicle’s battery via the ignition switch. Diode D1 provides reverse polarity protection, while REG1 provides a stable 5V supply for the ICs. Henri W. Klok, Algester, Qld. ($50) "Program Listing 'w2 'get speed reading 'w3 'store speed setting 'w4 'compare speed value  'start program for b0 = 1 to 4  'time to settle high 2    'after switch on pause 100   'green led flashing low 2 pause 100 next  'get speed setting again: w3 = 0 for b0 = 1 to 4    'take four readings pulsin 3,1,w2 if w2 >15800 then again   'prevent over run in w3 w3 = w3 + w2    'store speed setting next  'start to compare speed chkspd: w4 = 0 for b0 = 1 to 4    'again four readings pulsin 3,1,w2 if w2 = 0 or w2 > 15800 then error 'speed below range w4 = w4 +w2     'actual speed reading next if w4 > w3 then green 'below speed if w4 < w3 then red  'overspeed red: w5 = w3/20 w5 = w3 - w5  '5% of speed limit if w4 < w5 then alarm high 4    'just over speed go to chkspd error: for b0 = 1 to 4 alarm:  high 1    '5% more over  pause 500 high4    'speed limit   HIGH 2 low 2        Pause 500 pause 100       Low 2 low 1        next w5 = 0       w2 = 0 goto chkspd   'check again   goto chkspd green:  high 2    'turn on green low 4    'turn off red w5 = 0 goto chkspd   'check again

Low-cost low-intensity alarm

Ever needed an alarm to monitor something but you didn't want to annoy the neighbours or scare the hell out of the family in the middle of the night?

Using a feature found in most smoke alarms these days, this little circuit may do the trick for you. When the 9V battery level drops to about 7.4V in most smoke alarms, they emit a brief chirp at around 40 second intervals. In most cases, this is enough to attract attention, without the likelihood of waking the whole household.

In effect, this circuit simply powers the smoke alarm at 6.8V, as derived from a zener diode (ZD1), so that it sounds its low battery warning. Apart from the zener diode, the smoke alarm draws only a few microamps, with a brief current spike during the "chirp".

Smoke alarms are now very cheap to buy so this is a good alternative to a piezo siren.

More information on a common Smoke Alarm chip made by Motorola can be obtained at:

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