Price - Stock No 560

Price (with Buzzer) - Stock No 561

PCB Layout


Two clever curcuits that give a reminder to fasten your seatbelt.  

Construction Method

1. Check your P.C.Board carefully for damage to the tracks. An Electronic Circuit Tester or a Multimeter will help check that there are no broken tracks.

2. Identify the resistors by their colour bands or with a multimeter. Bend the legs with long-nose pliers to match their holes and push them into place against the surface of the board. Remember that the drawing is looking at the PLAIN side with the tracks away from you. Bend the legs sideways a bit to stop the resistors from falling out.   Solder in place - see Soldering Technique

3. Two PCB pins are located at the +12V and 0V points.  If you are constructing the Seatbelt reminder with buzzer, two additional PCB Pins are to be placed at the connection points for the buzzer.  Solder in place.

4. The 10uF electrolytic capacitor is polarised. Look at the body of the cylinders to find the values and check their locations from the drawing. On the cylindrical body there is also an arrow pointing down one leg of the capacitor. This is the negative leg. It won't work if it is in the wrong way round.  The 0.1uF capacitor is not polarised, insert as shown.  Solder both in place.

5. The BC 558 and the 2N7000 have a flat on the body. Mount them with the flats as the drawings show.  Care should be taken when soldering the transistors - excessive use of heat from the soldering iron may permanantly damage these components. 

6. The LED is also also polarised. There is a flat on the small flange at the base of the globe. This is above the K leg, which is also the shorter leg. Put the flats as the drawing shows.  Solder in place.

7 . If you are constructing the Seatbelt Reminder with buzzer, connect the buzzer taking care to connect the red lead of the buzzer to the +ve connection point.

8. A solder lug is supplied for the 0V connection to the vehicle and a 'wire tap is used to connect to the vehicle's accessory / ignition.  Note the +12V connection is to be connected to the vehicle's electrical system, 'wire tapped' to a wire that shows +12V when the ignition/accessory is switched on, this can generally be found coming from the vehicle's ignition switch.  One way to locate such a wire is to use a multimeter and a sewing pin - connect the multimeter's black wire to the chassis (0V).  Push the pin (and being careful not to prick yourself) into the wire to be tested and connect the red lead of the multimeter to the pin.  If +12V appears when the ignition is turned on and 0V when it is off, you have found a suitable site for the wire tap.  If not select another wire from the ignition and test again.  The wire tap is fitted with a pair of pliers.  Check that you have made the connection positivly by using a multimeter on the +12V and 0V points on the circuit board. 

Technology Notes

Lets first look at the seatbelt reminder (without buzzer).  When the ignition is turned off, the 10uF capacitor and the 0.1 uF capacitor are empty (think of them like a bucket - electricity is water.  The 10uF capacitor is 100 times bigger than the 0.1uF capacitor).  When the ignition is turned on, the 0.1uF capacitor begins to fill via the 100K resistor, while the 0.1uF capacitor is less than full it turns the BC 558 on allowing the 10uF capacitor to fill via the 100 R resistor.  A very short time after the 0.1uF capacitor fills to a point that it turns off the BC 558.  In short, the 0.1uF, 100K and BC 558 work togeather to fill the 10uF capacitor and then turn off in a split second.  See - Transistors

Now that the 10uF capacitor is full, this allows the 2N7000 to turn on and illuminate the LED via the 470R resistor.  Over a period of about a minute, the 10uF capacitor empties via the 680K resistor, when a certain level is reached, the 2N7000 turns off, extinguishing the LED. 

When the ignition is switched off, the 0.1uF capacitor is emptied through the 10K resistor - ready for re-use. 


The addition of the buzzer in the 'seatbelt reminder with buzzer' gives a more prominant reminder.  The mechanics are the same as above with the addition of a second BC558 to handle the additional drain of the buzzer.  This BC 558 is driven from the 2N7000 via a 470R resistor.  A 100K trimpot is incorporated to allow volume adjustment.

Have fun and go safely.

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