This weekend I did something I thought I really would never do – I already knew that this circuit was a loser. The guy has a web page with “the simplest LED flasher”, which uses an NPN transistor with the base disconnected and the emitter positive which is supposed to break down at less than 12V. This is a ‘negative resistance’ which causes the capacitor to discharge and flash the LED. This uses the transistor in a way that it was never meant to be used. I think it’s an unacceptable design – I’ve never seen this used in any commercial product or hobby kit. But I decided to try it with three different transistors.
I built the circuit as shown in the diagram, but I used a 470 uF capacitor. One of the transistors let the LED stay on all the time. The second one – a PN2222A – flashed, like it was supposed to. The third one did nothing, even when I turned the voltage up to 30 volts. Only one out of three attempts worked, which is not an acceptable success rate.
But the last one really had me puzzled. I subjected the transistor to up to 31 volts yet it did not break down, and that is not normal – almost all of the transistors should breakdown a few volts above their 5 or 6 volt maximum emitter to base breakdown voltage rating. This transistor is one I recently purchased, a run-of-the-mill transistor, the 2N3904. What did they do to it to make the thing so immune to breakdown? If all the newer transistors are like that, this circuit would never flash (see note below).
The transistor that let the LED stay on constantly was behaving like a zener diode. When the voltage reached a certain point, the transistor broke down and started to conduct, and let the current through to the LED, which to my thinking is normal. It’s just like a zener diode.
The transistor that flashed was exhibiting the negative resistance that let the voltage build up, then discharge through the LED. I varied the voltage from below 11 volts up to 12V and the flash rate was speeding up faster as it approached 12V. The circuit is very sensitive to voltage, but will the next transistor cause the circuit to be sensitive to a different voltage range? Or will the circuit just not work, like two of the three I built. It’s best to avoid this circuit and not slip into the pitfalls of a really bad design.
Note: I’m really puzzled about this, and wondering if the transistor chips are being made with state-of-the-art stuff like those found in more advanced chips. It’d make sense if the chip fab plant used the higher tech stuff to make transistors such as the 2N3904 transistor that I tested. This transistor was a simple design first made by Motorola in the early to mid 1960s. Any transistor maker can make a 2N3904 if it meets the minimum specifications in the datasheet. But there’s nothing from stopping the maker from far exceeding those specifications.
Think about this. A transistor could have some extra “enhancements” built into the silicon that is not a part of the actual transistor. This could monitor the temperature. for example, and shut the transistor down if the temperature exceeded a safe margin. Or if you want to get really paranoid, it could monitor the data, looking for secret messages, such as “Bin Laden” or similar. Just kidding, but the silicon could really add some safety features to the transistor. Better not tell anyone, though. It’s a s-e-c-r-e-t. 😉
