Hi - Some might recognise this as a continuation of my prob with schmitt-triggering. With your help I got over that fence, thanks, but then ran smack into another one
I used a CD40106B hex schmitt inverter, as in the pic, and everything seemed fine.
(a) When the input voltage is below about 2V, the other led comes on and the BD139 conducts.
(b) When the input voltage is over about 3V, one led comes on and the BD139 conducts.
When I replace R9 with a relay (with a diode across it), (a) above still works fine, with the relay clicking nicely, but with (b) the led flickers and the relay buzzes until the voltage is high enough, when they both settle. It's as if the schmitt action of that section of the CD40106B isn't happening.
Stop press! To add insult to injury, while checking the behaviour for this post, I found that now when I replace the relay with the resistor again, it no longer wotks fine. The led flickers.....
I used a CD40106B hex schmitt inverter, as in the pic, and everything seemed fine.
(a) When the input voltage is below about 2V, the other led comes on and the BD139 conducts.
(b) When the input voltage is over about 3V, one led comes on and the BD139 conducts.
When I replace R9 with a relay (with a diode across it), (a) above still works fine, with the relay clicking nicely, but with (b) the led flickers and the relay buzzes until the voltage is high enough, when they both settle. It's as if the schmitt action of that section of the CD40106B isn't happening.
Stop press! To add insult to injury, while checking the behaviour for this post, I found that now when I replace the relay with the resistor again, it no longer wotks fine. The led flickers.....
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I really couldn't find any reason for the failure - I presume it is the 40106 though Graham has not confirmed that fully. In the specs the 40106 can drive 10ma per output so though I recommended he increase the resistors on the outputs, it should have coped OK as it is shown. And as the emitters of both BC547s are on the base of the BD139, they should not go above 0.6V which is well within the reverse BE voltage limit even if the bases are grounded. As the problem started when Graham put the relay in, I'd guess that has some bearing on it, maybe the diode was not properly connected (I've done that :-)Graham - look forward to your further feedback.
