Wiring & diagram Info

The author wanted a very cheap and simple alarm for some of his possessions, such as his electrically assisted bicycle. This alarm is based on a cheap window alarm, which has a time-switch added to it with a 1-minute time-out. The output  pulse of the 555 replaces the reed switch in the window alarm. The 555 is triggered by a sensor mounted near the front  wheel, in combination with a magnet that is mounted on the spokes. This sensor and the magnet were taken from a cheap bicycle computer. 

Cheap Bicycle Alarm Circuit Diagram

The front wheel of the bicycle is kept unlocked, so that the reed  switch closes momentarily when the wheel turns. This  triggers the 555, which in turn activates the window alarm. The circuit around the 555 takes very little current and can  be powered by the batteries in the window alarm.  There  is just enough room  left inside the enclosure of the window  alarm to mount the time-switch inside it. 

The result is a very cheap, compact device, with only a single cable going to the reed switch on the front wheel. And the noise this thing produces is just unbelievable! After about one minute the noise stops and the alarm goes back into standby mode. The bicycle alarm should be mounted in an inconspicuous place, such as underneath the saddle, inside a (large) front light, in the battery compartment, etc.

Hopefully the alarm scares any potential thief away, or at least it makes other members of the public aware that something isnt quite right.

The IC is a quad 2 input “AND” gate, a CMOS 4081. These gatesonly produce a HIGH output, when BOTH the inputs are HIGH. Whenthe key wired to `E` is pressed, current through R1 and D1switches Q5 on. The relay energises; and Q5 is `latched on` byR8. Thus, the Alarm is set by pressing a single key, say one ofthe tw1o non-numeric symbols.The alarm will switch off when the 4 keys connected to“A,B,C,D” are pushed in the right order. The circuit worksbecause each gate `Stands` upon its predecessor.If any key otherthan the correct key is pushed, then gate 1 is knocked out of thestack, and the code entry fails. 

Pin 1 is held high by R4. This`Enables` gate 1; and when button `A` is pressed, theoutput at pin 3 will go high. This output does tw1o jobs.It locksitself `ON` through R2 and it `Enables` gate 2, by taking pin 5,high. Now, if `B` is pressed, the output of gate 2, at pin 4will go high. This output does tw1o jobs. It locks itself `ON`through R3 and it `Enables` gate 3 by taking pin 12 high.Now, if `C` is pressed, the output of gate 3 will lock itself`ON` through R5 and, by taking pin 8 high, `Enable` gate 4.Pressing `D` causes gate 4 to do the same thing; only this timeits output, at pin 10, turns Q4 `ON`. 

This takes the base of Q5to ground, switching it off and letting the relay drop out. Thisswitches the alarm off.Any keys not connected to `A B C D E` are wired to the base ofQ1. Whenever `E` or one of these other keys is pressed, pin 1 istaken low and the circuit is reset. In addition, if `C` or `D`is pressed out of sequence, then Q2 or Q3 will take pin 1 low andthe circuit will reset. Thus nothing happens until `A` ispressed. Then if any key other than `B` is pressed, the circuitwill reset.Similarly, after `B`, if any key other than `C` is pressed,the circuit will reset. The same reasoning also applies to `D`.The Keypad needs to be the kind with a common terminal and aseparate connection to each key. On a 12 key pad, look for 13terminals. The matrix type with 7 terminals will NOT do. 

Wire thecommon to R1 and your chosen code to `A B C D`. Wire `E` to thekey you want to use to switch the alarm on. All the rest go tothe base of Q1.The diagram should give you a rough guide to the layout of thecomponents, if you are using a strip board. The code you choosecan include the non-numeric symbols. In fact, you do not have touse a numeric keypad at all, or you could make your own keypad.I haven`t calculated the number of combinations of codesavailable, but it should be in excess of 10 000 with a 12 keypad; and, after all, any potential intruder will be ignorant ofthe circuit`s limitations. Of Course, if you must have a moresecure code, I can think of no reason why you shouldn`t addanother 4081 and continue the process of enabling subsequentgates. Or you could simply use a bigger keypad with more “WRONG”keys.Any small audio transistors should do. 

The 27k resistors couldbe replaced with values up to 100k. And the only requirementsfor the 4k7 resistors is that they protect the junctions whileproviding enough current to turn the transistors fully on.Capacitors (C1 C2 C3 C4 C5) are there to slow response timeand overcome any contact bounce. They are probably unnecessary.

Alarm Control Keypad Circuit Diagram

The Keypad must be the kind with common terminal  & 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. On the Support Page you will find details of how to Make Your Own Keypad.

The relay is energized by pressing a single key. Select the key you need to make use of and connect it to terminal E. Select the keys you need to make use of for your security code and connect them to A B C & D. Wire the common lead to R1- and all the remaining keys to F.

When you press E the relay energizes & the 12-volt output moves from the off to the set terminal. The green LED also lights. It provides  visual indication that the alarm is set.

When you press keys "A B C and D" in the right order - the relay de-energizes - & the 12-volt output returns to the "off" terminal. The green LED is also extinguished - to indicate that the alarm is switched off.

The remaining keys - those not wired to "A B C D and E" - are connected to "F". Whenever of these "Wrong" keys is pressed - the tried code entry fails - and the code entry sequence is reset.

With a 12-key pad - over ten 000 different codes are available. In the event you need a more secure code - you could basically use a bigger keypad with more "Wrong" keys wired to "F". A 16-key pad gives over 40 000 different codes. In the event you make a mistake while entering the code - basically start again.

The Support Material for this schema includes a step-by-step guide to the construction of the schema board - a parts list - a detailed schema description - and more.

The same thing happens if "C" or "D" is pressed out of sequence. If "C" is pressed before "B" - or "D" is pressed before "C" - the tried code entry will fail. And the code entry sequence will reset.

This electronic schema is a very nice and simple alarm schema that is based on the 555 timer integrated schema. This alarm schema can be used with many types of sensors like : light or temperature sensors. As you can see below some types of sensors that can be connected at this type of alarm schema ( darkness , light , cold and heat sensor ).

If the sensor detects that the temperature or light is outside of the desired range the alarm will be activated .By turning the  potentiometer from the base of Q1  you will modify the sensibility of the sensor . When the alarm is activated the Q2 transistor acts as an audio amplifier and speaker driver for the 16 ohms speaker .

Dual Sensor 555 Timer Alarm Circuit Diagram

This alarm schema require a 12 volts DC power supply schema .