# Exams, Need help with circuit.

Discussion in 'Homework Help' started by MrAdam, Jun 10, 2010.

1. ### MrAdam Thread Starter New Member

Jun 10, 2010
2
0
Hi everyone. I don't know if this is the correct place to ask this, but i am gonna give it a go.
I have my finals exam tomorrow and will have to explain a circuit which was handed out a couple of days ago. I understand most of it, but i will have to present it for about 20 minutes and i don't feel that i got enough information to do that. So i was wondering if any of you could help me explain the attached circuit more specifically?

Regards, Adam.

Here is the text from the paper which was handed out:

"This circuit (Fig. 1), used in conjunction
with a thin piezoelectric plate,
senses the vibration generated on
knocking a surface (such as a door or a
table) to activate the alarm. It uses readily available,
low-cost components and can
also be used to safeguard motor vehicles.
The piezoelectric plate is used as the sensor.
It is the same as used in ordinary
circuit. When someone knocks on the
door, the piezoelectric sensor generates
an electrical signal, which is amplified
by transistors T1 through T3.
The amplified signal is rectified
and filtered to produce a low-level
DC voltage, which is further amplified
by the remaining transistors. The
final output from the collector of pnp
transistor T6 is applied to reset pin 4
of 555 (IC1) that is wired as an
piezobuzzers and is easily available in the
market.
The piezoelectric plate can convert any
mechanical vibration into electrical variation.
As it doesnt sense sound from a distance
like a microphone, it avoids false
triggering. The plate can be fixed on a door,
cash box, cupboard, etc using adhesive. A 1-
1.5m long, shielded wire is connected between
the sensor plate and the input of the
astable multivibrator. Whenever the collector
of transistor T6 goes high, the astable
multivibrator activates to sound an alarm
through the speaker. The value of resistor
R12 is chosen between 220 and 680 ohms
such that IC1 remains inactive in the absence
of any perceptible knock.
When the circuit receives an input signal
due to knocking, the alarm gets activated
for about 10 seconds. This is the
time that capacitor C5 connected between
the emitter of transistor T4 and ground
takes to discharge after a knock. The time
delay can be changed by changing the
value of capacitor C5. After about 10 seconds,
the alarm is automatically reset.
The circuit operates off a 9V or a 12V
battery eliminator."

2. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
A good starting point would be for you to write up your explanation of the circuit's operation at this point.

hgmjr

3. ### jpanhalt Expert

Jan 18, 2008
6,949
1,453
To help in that, you might consider the circuit in two parts. One part is T6 and everything to its right. The other is what's left. That division is also made in the explanation you received. Consider a creating a topical outline. Then start filling it in. In particular, you need to discuss the piezo sensor part of the circuit. The 555 part is pretty obvious.

I would not worry so much about filling the 20 minute period as being able to limit a meaningful presentation to just 20 minutes. But as just said, the first step is getting started.

John

4. ### MrAdam Thread Starter New Member

Jun 10, 2010
2
0
So far my idea was to start explaining the functions of a 555 timer component, and then the functions of an astable multivibrator and how it can be used to generate signals for a speaker. then i would talk about the piezo sensor, and how it functions. Both as a input and output device. And finally the transistor part, and this is where my main problem is. The transistor part of the circuit confuses me, i am not quite sure how to explain it.

5. ### jpanhalt Expert

Jan 18, 2008
6,949
1,453
Entire books have been written on the 555. Are you sure you can explain it AND the sensor part in just 20 minutes? Ask your self, what does the circuit do? What is its purpose? What in its design makes it suitable for that purpose?

Is the most important part of the circuit the switch that sounds the alarm or the part that detects vibrations? In my opinion, almost any switch will work, but what alternatives are there for the vibration sensor?

Ultimately, the decision has to be yours, as you are the one who attended the classes. Was the class about multivibrators/oscillators or general analog electronics and interfaces?

As for the transistor part, I would just walk through it. That is, what happens when with the signal from the piezo goes to the base of T1. Why are C1 and C2 there, and so forth?

John

Last edited: Jun 10, 2010