Neon glow bulb is a nostalgic decorate lamp everybody seems to love. A small version is used still mainly as neon indicator light, also called pilot light, in electric/electronic instruments and appliances as a neon glow bulb generally have a very long life, low current drain and is resistant to shock and vibration. Nowadays, neon glow lamp is experiencing a comeback as an architectural and artistic element, however, independent testing of a neon glow lamp is somewhat difficult because it obviously needs a high-voltage AC supply for the smooth run. Presented here is one battery-operated circuit of a simple and compact regular neon glow bulb (NE-2 style) tester wired around a couple of easy to get inexpensive electronics components.
As can seen in the circuit diagram, the design is centered on a NE555P (IC1) timer chip. The 555 timer is now ubiquitous, available on nearly any street corner for just a few coins. Next key component is the good old but quite popular 1300:8Ω small audio output transformer (TFR1) wired here in reverse mode (S:P) like a “tickle stick” inverter-type transformer. IC1 is configured here as a high-frequency pulse generator runs near 6KHz, and TFR1 is driven by its output through a small NPN transistor S8050/SS8050 (T1). RC components R1 (1K2) and C1 (100nF) sets IC1’s oscillator frequency. The entire circuit can be powered from one common 6F22 9V “transistor radio” battery by the push-to-on (momentary) test switch (S1). Note that there’s also one 1nF ceramic capacitor (C3) in series with the 2-pin neon glow bulb test socket.
Overall current consumption of the ‘loaded’ tester at 9V dc input is near 200mA. When Vcc is 9.0V, an ordinary DVM will register around 4.6V on the output pin (pin 3) of IC1 (~ 6KHz/60%). Random oscillograms captured while testing a small amber neon glow bulb is included here merely as a reference. You can see output across TFR1’s secondary (left) and output across the neon glow bulb (right) there.
You can ofcourse power up the tester from a 5V dc supply (instead of the 9V), for instance by a mobile phone power bank or breadboard power supply. Refer following table to see the voltage and frequency drifts, as observed in my breadboard prototype.
|DC INPUT||IC1 Pin 3 VOLTAGE||IC1 Pin 3 FREQUENCY|
Note over again that the inverter transformer used here is a 1300: 8Ω (ac impedance) small audio output transformer wired in reverse i.e. its original primary is used as the secondary and vice versa. Usually such transformers has a primary coil dc resistance more than 80Ω and around 3Ω for the secondary coil (or so). The primary part is usually marked by a small blob on the top of the bobbin, however, you can use a digital multimeter to key out both the primary and secondary coils. Also take note, the inverter transformer will very quietly sing at 3-6 KHz, that’s normal, but do not touch the output leads of the inverter transformer (test socket). You have been warned!
And, to a couple of quick snaps from my workbench: