If you are of a sensitive disposition, get bored easily, or have a short attention span…Look away now…It’s going to get technical… And I’ll try to address Minx and Fruitcake’s concerns while I’m at it…
Not happy with altering the value of the potentiometer, I decided to continue down the route of the heating up of the resistor affair…
A good way to have a visual look inside an electronic circuit is to connect a milliamp meter in series with the battery and study the varying power consumption. It would certainly tell if the circuit was experiencing ‘Thermal Runaway’…
The milliamp meter showed an ambient consumption of 21.7 mA…But continued to rise after a short period of time to 23.5 mA. This rise in current required adjustment of the potentiometer which brought the consumption back to 21.7 mA. Over the period of ten or fifteen minutes the consumption had risen so much that there was no longer any more adjustment on the potentiometer, and I was no longer able to stabilise the circuit.
What to do…?
It was obvious that ‘thermal runaway’ was the culprit, but after the heatsink failed to show any results I removed the circuit board from the unit, and replaced the 680k ohm ( what should have been a 2 watt resistor) with three large resistors (the only heavy duty ones I had) in series, and making up the 680k ohms required. A bit untidy, but practical…
After reassembly of the unit, it was ready to test. :surprised:
I ran the unit under lab conditions for several minutes, where an initial rise in consumption was counteracted by adjustment, until it stabilised. Good news, it maintained the 21.7 mA for over twenty minutes, and following a search of the front and rear lawns, did not require further adjustment of the potentiometer. No gold coins though…
It had passed it’s test with flying colours using the inbuilt 8 ohm speaker, however, when I repeated the test around the garden using the headphones the sensitivity became meteoric! and I found a few anomalies that had laid undiscovered while using the speaker.
Some underground cables connecting the summerhouse to the garage, and even some metal shuttering I had used while concreting the drive…:shock:
So it’s all systems go folks…
Minx! Even with a fully operating metal detector I’m not expecting to exceed the remaining space in my rucksack with artifacts… I must point out also, that with an ambient current of 21.7 mA, six AA batteries should be quite sufficient for a days detecting. But I admire your optimism…
Fruitcake! Unfortunately (or fortunately as the case may be) I don’t have any access to Sky. The only dish in my house is the one that I eat my porridge out of…:-p
I watch very little TV, I find it terribly counter productive, however…After suffering a couple of heart attacks I tend to get very tired in the evenings, and pretending to watch TV while resting stops Mrs Fox from worrying and getting concerned about my health…