# Need Help on a Simple Time Delay Circuit

#### dcodner

Joined Nov 21, 2007
3
Hey guys I just found this site. I am having trouble with my Project for school. I have to design a burglar alarm circuit. The requirements are: must not exceed 9volts, alarm must sound 10secs after it is triggered and go off for 10 sec. It must be cheap like under $20-$30 to construct. And no 555 timers may be used.
So far I have a 9 volt battery, 1 - 1M Ohm Resistor in series with a 10-micro farad Capacitor. My siren is connected across my capacitor. and i have a switch at the positive terminal of the battery ran in series with the resistor. but every time i hit the switch i get no delay. Please help.
D

#### dcodner

Joined Nov 21, 2007
3
any ideas would really be appreciated

#### GS3

Joined Sep 21, 2007
408
And no 555 timers may be used.
Dayum!

Well the 555 just contains a couple of comparators and a couple of other things which you can replicate. You don't even need all of it. Just replicate what you need.

#### thingmaker3

Joined May 16, 2005
5,084
Try a Google search on the word "monostable." You will find many circuits which do not require a 555.

#### dcodner

Joined Nov 21, 2007
3
How do I replicate a 555 timer? What components do I need and how do i put them together? I have no idea how to out everything together. In class we only learn about theory and impractical applications.

Also i did a Google search on "monostable" and I got almost all hits about monostable vibrators (have no idea what that is).

#### GS3

Joined Sep 21, 2007
408
How do I replicate a 555 timer? What components do I need and how do i put them together? I have no idea how to out everything together. In class we only learn about theory and impractical applications.

Also i did a Google search on "monostable" and I got almost all hits about monostable vibrators (have no idea what that is).
You can find out about monostables at http://en.wikipedia.org/wiki/Multivibrator

#### Voltboy

Joined Jan 10, 2007
197
You could use a high capacitance capacitor and connect it to the negative of the comparator and when it charge it keep the comparator on and keep discharging, remember you must use a voltage divider in the comparator positive input of the comparator. Connect the output of the comparator to a flip flop.
I'll recommend a 1000uf capacitor with 10k resistor.
I'm not sure if this will work but you could try

#### GS3

Joined Sep 21, 2007
408
I have to design a burglar alarm circuit. The requirements are: must not exceed 9volts, alarm must sound 10secs after it is triggered and go off for 10 sec.
OK, let us see. Here's one possible way of doing it:

Trigger switch closes and triggers a monostable M1 of T=20 sec and another one M2 of T=10 sec. Alarm sounds when M1 is on and M2 is off.

A monostable can be easily built with a capacitor and a resistor and a comparator.

Another way of doing it could be with a single capacitor charging and two comparators so that the output is true only between two voltage levels:

Capacitor starts charging from 0V
Between 0 and X volts alarm is silent.
Between X and Y volts alarm sounds
When voltage reaches Y volts system resets to 0V