IAN LANG ELECTRONICS

A Level Electronics

AQA Unit 1- Introductory Electronics

1

A student designs a flood warning system. When the humidity goes above a set level,
a signal is produced that can only be stopped by operating a reset switch manually.
The signal activates a low power pulsing signal that is used to operate a high power
siren.

1 (a) Draw a system diagram by choosing appropriate input, process, and output subsystems
from the list below.

astable audible warning device comparator

driver humidity sensor latch

reset switch set level

(8 marks)

Well, just like any examination question, look at how many marks are available in the bottom right hand corner. It's eight, so they want you to do eight things. There are eight subsystems named, and the conclusion then is that they want you to organise them into a system that works the way as described in the question. First thing then, mark off what each does in pencil. The astable provides an on-off pulse. The audible warning device is the siren. The comparator can compare one voltage with another. The driver is the bridge between the control circuit and the siren. The humidity sensor is obvious, and the latch is the subsystem that's going to keep the siren going until you press the reset switch. The set level is the reference voltage.

So, we know that the humidity sensor will output a voltage and that voltage will depend on how humid it is. We also know that the set level is a constant voltage. Look at the question again: "When the humidity goes above a set level,a signal is produced" it says. So, we need to compare the constant voltage with that which the humidity sensor is putting out. We need a comparator.
The question is asking for a system diagram, not a schematic. All we have to do is draw boxes and connect them. We know that we need to feed both voltages to a comparator. Our diagram then, begins like this:

Alright then, so we know this comparator will output a signal whenever the output voltage is greater than the input. So far we've done this part of the question:

"When the humidity goes above a set level,a signal is produced"
The question continues:

"that can only be stopped by operating a reset switch manually."

Right, so we need to lock it down. The comparator on its own will not do this; if the humidity goes down again the output will drop and if it goes down far enough the comparator output won't be high (i.e. there'll be no voltage at the output and thus no signal. The obvious candidate for this is a latch. So:

Once the latch has activated, the only way to deactivate it again is to starve it of current. For this we need that reset switch:

It goes in the latch because switching the current off anywhere else is not going to starve that latch and it'll just keep right on conducting. Five down and three to go. What's the next part of the question:

"The signal activates a low power pulsing signal"

Oh does it now? Right then we need that astable connected to the latch then:

Right, now we're cooking. We've got a low power pulsing signal up and running, let's look at that last bit of the question again:

"The signal activates a low power pulsing signal that is used to operate a high power
siren."

So, we need a bridge to come between the low-power pulse and the higher power siren. It's the driver:

And now it's pretty obvious where the audible warning device goes:

So, just to make sure we know what we're doing let's go through our system step by step. The comparator's got two inputs, the sensor and the set level (which is a constant fixed voltage). If the voltage output of the sensor is not greater than the fixed, there's no output. If it is, output goes high. If output from the comparator goes high, the latch activates.

The latch keeps on conducting unless we stop it by hitting the reset switch. When the latch is conducting it provides power to the astable, which pulses. This switches the driver on and off, which does the same to the siren. Bingo! We cracked it!

Rather nastily they've tagged a rider on to this. Here it is:

Here's an easy three marks as long as you know your components. They're looking for the fact that you know and realise:
1. A potentiometer can be used as a voltage divider and therefore you can use it to change the set level.
2. An op-amp can be used as a comparator.
3. A MOSFET is a kind of transistor and can therefore be used as a switch. The driver switches the siren off and on.

So, you write down:

Set Level

Comparator

Driver

And you're off to a flying start with eleven easy marks.

Often they'll use some other sort of sensor. They're all exactly the same principle: they put out a measurable voltage which you can compare against a fixed reference. If they use the term relay as above, it's the latch because starving the coil of current deactivates the latch, and the same goes for an SCR or as it's sometimes called a thyristor.