Hello, I am new to this site and loving it. I did a bit of Electronics in 1995 where I made a flashing led circuit with 2 led's and it was cool. My other hobby is model trains and I would like to put 2 small led's in buffer stops and make them flash and run it off a DC train controller. Does anyone have a simple circuit that would do the job for this? I am basically a noob at electronics but I can follow a simple circuit.
Thank you.
Gary.
Although datasheets always show it as either 5V or 6V, in reality reverse breakdown of the base-emitter junction can be a good bit higher than that - more like 7 or 8V. It's exactly the same effect as a 'zener' diode of more than 3 or 4 volts, merely the reverse breakdown of a pn junction by avalanching. The difference is that with an actual zener it's designed to cope with a certain power level and is properly characterised for voltage. With the transistor, it will start to do damage that will probably affect the transistor properties.
On a 9V battery you can usually get away with it without problems, although technically you are outside the datasheet. With higher voltages, traditionally people used to put diodes in series with the emitters like this
That's from Transistor Circuit Design published by the engineering staff of Texas Instruments in 1963.
I'm not sure why that's better than having the diode in series with the base where you'd preserve the output swing. The effect on the timing would be the same. Perhaps there's a snag with doing that that I can't see.
ps: there is always an advantage to being awake at 4am 
