Need Help- Burglar Alarm System

Question

I am making a burglar alarm system for my high school project. It uses a PIR sensor, a piezo buzzer, switch, battery, transistor, resistor and an LED. However, I have a 9V battery and a MPS2 222 AAAA transistor instead of the recommended BC 547 one for these kinds of projects. I wanted to know if I can still use the MPS2 transistor with a 9V battery and what will be the recommended resistance for the circuit?

Answer 1

For driving simple loads, both of those transistor can be used. And it causes no change to the resistor.

Answer 2

A typical 1" diameter Piezo buzzer draws about 1 mA per volt so if rated for 9V it draws about 8 mA. These tend to be higher pitch than magnetic buzzers which draw more current.

Using Ohm's Law that means the Piezo is almost similar to a 1 kohm resistor 9V/8mA = 1.12 k = V/I So you may not even need a transistor as an inverting or "low-side" switch. So it is redundant but OK to keep if you want a louder buzzer later.

When using transistors as switches the current gain drops to about 10% less when defined as a "saturated" switch or Vce(sat)= ___ @ ___ mA. Again using Ohm's Law we see both the MPS2222A and the BC547 Vce(sat)/Io=Rce is < 10 ohms e.g. 90mV/10mA= 9 Ohms. This is also about the resistance of an inexpensive 9V alkaline battery and both are much less than 1 kOhm so both will not lose much voltage or get warm.

MPS2222A BC547 Datasheet

If you look at the tables of specs. in the datasheets for Vce(sat) in both transistors, you will notice a wide tolerance for Vce(sat) and the base resistor depends on using a standard current ratio of Ic/Ib=10 and 20 respectively or 50 in the case of superbeta transistors.

But as a ballpark estimate, if both input and output voltages come from the same, this means as a switch the base R can be about 10x the load equivalent resistance (just over 1k). That means the base could be 10k. But 1k is only 9V-0.7=8.3V or 8.3 mA is not excessive so this overcurrent will not affect the result except wasting battery current. So 1k works too or any value in between but 10k is the preferred value. This will also still work if your buzzer takes 20mA using the MPS2222A but start to drop more voltage if the load is much higher.

This is all based on the perhaps false assumption that you were told the PIR could switch 1k. Many of the so-called "digital" PIRs use a FET source and recommend 47k load to ground.

Thus you may have to change to a different switch!!