Lighting a LED with a candle flame
After reflecting on the circuit 'oscillator running on home made thermocouple', and failing to get a LED lighting up yesterday, it all fell into places, and I realized that the means to obtain the required 2V DC for the LED WAS ALREADY THERE.
For the curious and experimenters, here is the complete diagram:
Please, this is a BIG file, >3 MB
And this is what the very simple circuit looks like:
Scope is on source, to avoid loading.
This home made thermocouple is really not suited for temperatures above 200 C, not only will the solder melt, but in the top of the candle flame it will simple burn up (> 1000 C there). In the bottom of the candle flame you will be OK for a short while, better is to get perhaps a type E thermocouple, more voltage an it better stands high temperatures.
The LED is not very bright, and it is bright daylight here, perhaps tonight I will try to make a youtube video or better pictures with the LED visible.
The secret to get the LED on, is in fact very simple. The waveform at the gates of the JFETs is limited by the gate source diode junction conducting (max about 1.4 Vpp). So in fact there is already some gate current. Re-directing the gate current through the LED, and decoupling the LED, and moving it so it is ground connected, is all that is required. The voltage over the LED slowly rises as the gate current charges the 100 nF capacitor, and at about 2 V all current goes into the LED, and it lights up.