|
THE AMMETER
The ammeter is placed in SERIES with one lead of a circuit. It must
be placed around the correct way so the needle moves up-scale.
An ammeter is really a microamp-meter (it's called a movement -
generally a 0-30 micro-amp movement) with a SHUNT (a thick piece of
wire) across the two terminals.
To cover the range of current used in electronic circuits, there are
basically 3 types of amp-meters (or 3 ranges):
0 - 1 amp (0 - 1A)
0 - 1milliamp (0 - 1mA)
0 - 1 microamp (0 - 1uA)
In each range you can get many different scales, such as:
0 - 1A, 0 - 10A, and higher
0 - 10mA, 0 - 100mA, 0-250mA, 0-500mA
0 - 1uA, 0 - 100uA, 0 - 500uA
(0 - 1uA uses a 1uA movement)

Connecting an AMMETER
An ammeter is never connected
across a battery or the supply rails of a project as this will
create a SHORT-CIRCUIT and a large current will flow to either
burn-out the meter or bend the pointer.
However, you need to know which way to connect a meter so
that it reads up-scale.
This is how you do it:
Remember this simple fact: Current flows through the meter from the
+ve lead to the -ve lead and this means the leads must be placed so
that the positive lead sees the higher voltage.
Do not place an ammeter ACROSS a component. This will generally
cause damage and in most cases it will not tell you anything.
You can check to see how much current is flowing through a circuit
by flicking one lead of the ammeter onto the circuit and
watching the needle. If it moves up-scale very quickly, you know
excess current is flowing and a higher range should be chosen. If
the needle moves fairly slowly up-scale, the chosen range may be
correct.
Always start with a high range (0-1Amp for example) and if the
needle moves a very small amount up the scale, another range can be
chosen.
DON'T FORGET: Placing an ammeter on a circuit is a very
dangerous thing because it is similar to playing with a jumper lead
and represents a lead with a very small resistance. It is very easy
to slip off a component and create a short-circuit. You have to be
very careful.
Ammeters have to be connected across a "gap" or "cut" in a circuit
and the easiest way to get a gap is across the on/off switch.
The animation below shows how to connect an ammeter:
|
THE
MICROPHONE
Basically there are two different types. One
PRODUCES a voltage and the other REQUIRES a voltage for its
operation.
This means you need to supply energy to the second type and
this is very important when you are designing a battery-operated
circuit and need to have a very low quiescent current.
Here is a list of different types of microphones and their
advantages:
SUPPLY VOLTAGE
REQUIRED: |
Electret
Microphone - sometimes
called a condenser microphone. Requires about 2-3v @ about 1mA.
Extremely good reproduction and sensitivity - an ideal choice.
Output - about 10 - 20mV |
Carbon
Microphone - also called
a telephone insert or telephone microphone. Requires about 3v -
6v. Produces about 1v waveform. Not very good reproduction. Ok
for voice. |
NO SUPPLY VOLTAGE
REQUIRED: |
Crystal
Microphone - also called
a Piezo microphone.
Produces about 20-30mV
Produces a very "tinny" sound - like talking into a tin.
|
Dynamic
Microphone - also called
a Moving-Coil, Moving-Iron, Magnetic Microphone or Ribbon
Microphone. Very good reproduction. Produces about 1mV.
A speaker can be used as a microphone - it is called a Dynamic
Mic. or Magnetic mic. - output about 1mV |
If a microphone produces
about 20mV under normal conditions, you will need a single stage of
amplification. If the microphone produces only 1mV under normal
conditions, you will need two stages of amplification.
The circuits below show the first stage of amplification and the way
to connect the microphone to the amplifier.

Connecting an electret microphone.
The 100n
capacitor separates the voltage needed by the microphone (about
1-2v) from the 0.6v base voltage. A good electret microphone can
hear a pin drop at 2 metres. A poor quality electret mic produces
crackles in the background like bacon and eggs frying.

The internal
construction of an electret microphone.
Air enters the
electret mic via the top holes and moves the thin mylar sheet. This
changes the distribution of the charges on the plastic and the
changes is passes down the Gate lead to the FET. The FET amplifies
the signal and the result is available on the Drain lead.

The "charged cell" is connected between ground and Gate.


Connecting a
Crystal microphone.
The crystal microphone has an almost infinite impedance - that's why
it can be connected directly to the base of the transistor.

Connecting a
Magnetic microphone.
The magnetic microphone has a very low internal resistance and needs
a capacitor to separate it from the base of the amplifying stage. If
it is connected directly, it will reduce the base voltage to below
0.7v and the transistor will not operate.
|
The
Super-Alpha Pair
(The Super-Alpha Pair is also called a Darlington
Transistor)
Two transistors can be connected as shown below to create a
Darlington transistor: |

The Flashing LED.
The Flashing LED has a chip inside the device to produce the
flash-rate. Simply connect the LED to a supply voltage (4v to 10v)
and the LED will flash at a rate of approx 2Hz. No external resistor
is needed up to 10v. For voltages higher than 10v, the resistor
should be 100 ohms for each volt above 10v.

|
|