Simple applications of neon glow lamps
The main voltage monitor
The simplest application is the main voltage monitor that is simply a
lamp that glows when the main voltage is present.
To obtain such a monitor it's enough to connect a resistor in series with the bulb
and connect at a main outlet. The resistance of that resistor may vary on the
type of bulb and the main voltage but it's not critical: about 150 KΩ for
220..230 Vac and about 39 KΩ for 110..120 Vac.
It's very important to choose a resistor that can support the desired voltage or
connect two or more usual resistors in series. The power of that resistor is quite
low and a 0.25 W is usually enough.
The telephone ring monitor
To light a neon glow lamp a relatively high voltage is necessary (about 70 V) and
this make the realization of a telephone ring monitor quite easy: the voltage of the
telephone line is usually less than 50 V and it's not enough to start the lamp.
When the phone rings an AC voltage of about 150 Vac is present on the line and the
bulb will glow.
The low current consumption of glow lamps is very useful in this case since it
doesn't overload the phone line.
The circuit is very simple and can be installed into a telephone plug.
- In certain countries the connection to a telephone line of non
certified devices (such as this one) is not allowed. This
application is only presented for didactic proposes.
- This project may require some modifications to adapt it to each
telephone standard, and may not work in some countries.
- If the lamp keeps glowing after the ringer, the maintaining
voltage of the lamp is too low (lower than the telephone idle
voltage). To solve this problem just connect two Zener diodes of
about 20 to 30 V in series back to back in series with the
With only a neon glow lamp and two resistors it's possible to build a useful tool
that checks if the earth wire of an AC outlet is correctly connected.
In facts the neutral wire and the earth wire are usually connected together in the
transformer and this connection short circuits the lamp that cannot glow.
If the earth wire is not connected or if the live and neutral wires are inverted
the full voltage would be present on the terminals of the lamp that would glow
indicating the presence of a problem.
This circuit is very simple and can be mounted into an AC plug as shown in the
This circuit can be adapted to 110 Vac mains by reducing the two resistors to
about 33 KΩ.
- This circuit is very simple and cannot detect every false connection;
for example if the neutral wire is not connected the lamp can't glow
even if there is a problem with the earth wire.
- This circuit only works correctly if the neutral is grounded at the
transformer (TN-S or TN-CS systems).
AC outlet tester
To improve the above circuit two more neon lamps are needed as well as some zener
diodes. The circuit is a little bit more complex.
Usually the neutral and the earth are at the same electrical potential and the two
resistors of 150 KΩ are in parallel making all three lamps glow. If a
connection is missing or is inverted the resistors will form a voltage divider that
reduces the voltage and makes one or more lamps dark. The two anti-series zener
diodes are there to avoid that the lamps glows with about the half of the main
Lowering the resistance of the resistors and the voltage of the zener diodes this
circuit can be adapted to 110 Vac.
This circuit is a little to complex to fit in a plug but it can easily fit in a
small plastic box.
The meaning of the three laps follows:
| lamp 1
|| lamp 2
|| lamp 3
|| No Earth
|| Live - Neutral inverted
|| No Neutral
|| Live - Earth inverted
|| No Live
Important remark: This circuit only works correctly if the neutral is grounded at the
transformer (TN-S or TN-CS systems).