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.
To everyone following commanderkelly initial blog, I must admit there are some strange goings on for such a simple circuit 
Like fmilburn I also breadboarded this to see how supply voltage affected the flash rate. I used 2N3804 transistors, 68k + 10uF/25v capacitors, plain old technology LEDs + 560 ohm current limiting resistors. These component values give a decent 1.22 Hz flash rate at 14v DC.
The flash rate did vary, and it increased as the voltage dropped to around that required by the LEDs...as expected. Mid-voltages I saw a flash rate as expected, as seen in the blue plot below. This was based on the following equation:
frequency = 1 / (0.38 * R * C)
= 1.06Hz
But at higher voltages the flash rate started to increase again and I wasn't sure why. I don't think it is to do with the capacitors: for some types of capacitor the capacitance value changes significantly with applied voltage, but that isn't the case for electrolytics. I ran the circuit without the LEDs and measured the frequency on an oscilloscope and it still exhibited this variance in frequency. That leaves the effect down to the transistors.
Focusing on the voltages on the transistor, and specifically the base voltage (which goes negative during switching and the capacitor voltage is maintained but inverts) this reverse biased the base-emitter junction of the transistor....there is a rated value in the specification sheets for that of 6v minimum. I suspect when that is overcome the junction behaves like a Zener diode, something we did not expect. Now the timing will start to vary as we see in the blue plots above. There is a solution though. Adding a simple diode like the 1N4148 in series with the base of both transistors prevents this effect occuring as these diodes now increase that breakdown voltage significantly to approx. 75v.
Plotting the frequency now gives the orange graph above. We can see the circuit behaves poorly at low voltages as it now has to overcome the forward bias of the new diode to start working. But at higher voltages it definately looks much better.
Rod
ps: there is always an advantage to being awake at 4am 
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.
