See he

http://tinypic.com/r/qsqp1z/9

Question is: why would you want to put a variable resistor between Z
and Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

On 10/11/2017 20:01, JoeJoe wrote:
See he

http://tinypic.com/r/qsqp1z/9

Question is: why would you want to put a variable resistor between Z
and Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

Adjust the temperature at which the LED comes on?
Presumably some kind of overheat warning device (assuming PTC sensor)?.

On 10/11/2017 20:01, JoeJoe wrote:
See he

http://tinypic.com/r/qsqp1z/9

Question is:Â* why would you want to put a variable resistor between Z
and Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

Well - if the component between Z and X is a thermistor
(temperature-dependent resistor), the variable resistor between Y and Z
lets you set the temperature at which the LED lights..... (I think).

On 10/11/17 20:19, Andy Bennet wrote:
On 10/11/2017 20:01, JoeJoe wrote:
See he

http://tinypic.com/r/qsqp1z/9

Question is:Â* why would you want to put a variable resistor between Z
and Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

Adjust the temperature at which the LED comes on?

I'd go with that. There's a thermistor below the variable resistor and
an LED triggered by the transistor.

Presumably some kind of overheat warning device (assuming PTC sensor)?.


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Change nospam to techie

On 10/11/17 20:01, JoeJoe wrote:
See he

http://tinypic.com/r/qsqp1z/9

Question is:Â* why would you want to put a variable resistor between Z
and Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

Because teh ciruit is so poorly designed obnly an adademic could have
desighed it

It allows variation in gain and so on of the transistor to be adjusted
for. For a single temperature anyway.

It is a circuit taht shouyld be thrown away though. Ther is no negative
feedback to compensate for tolerance, whatsoever.






--
€œBut what a weak barrier is truth when it stands in the way of an
hypothesis!€�

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Well, cannot see your circuit, but from others comments surely if you wanted
it to trigger at a temperature you would need it to suddenly come on at full
brightness not just come on slowly as the bias is altered.
I'd have expected some feedback in the system to make it happen like when
you need to make a relay go on hard.Schmidt Trigger or an IC that does
similar job.

Sorry I cannot spell today.
Brian

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This newsgroup posting comes to you directly from...
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"JoeJoe" wrote in message
o.uk...
See he

http://tinypic.com/r/qsqp1z/9

Question is: why would you want to put a variable resistor between Z and
Y, and not a fixed value one?

I could do this in my sleep years ago, honest...

In article ,
Brian Gaff wrote:
Well, cannot see your circuit, but from others comments surely if you
wanted it to trigger at a temperature you would need it to suddenly come
on at full brightness not just come on slowly as the bias is altered.
I'd have expected some feedback in the system to make it happen like
when you need to make a relay go on hard.Schmidt Trigger or an IC that
does similar job.

Easiest way is a comparator circuit using something like an LM311

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*I can see your point, but I still think you're full of ****.

Dave Plowman London SW
To e-mail, change noise into sound.

On Sat, 11 Nov 2017 07:26:44 +0000, The Natural Philosopher wrote:

It is a circuit taht shouyld be thrown away though. Ther is no negative
feedback to compensate for tolerance, whatsoever.

The biggest snag is that unless the temperature changes rapidly the
will be quite a band where the transistor is operating linearly and
the LED at variable brightness. Don't you need positive feed back to
make it switch cleanly from one state to the other? You then need to
add some hysterisis to suppress any tendancy ot oscilate just at the
switching point.

Recntly been round that loop with a circuit to swich a bank of IR
LEDs on when it gets dark and off when it's light. Ended up with
something similar, two transistors in darlington arrangement,
LDR/resistor potential divider on T1 base, 10K bias on the T1
collector T2 base connection, emmiters commoned then grounded via a
3R3 resistor, LED and current limit resistor and relay coil between
T2 collector to +V. Certain amonut of "adjust on test" value
selection to get the on and off points set. B-)

--
Cheers
Dave.

On 11/11/2017 08:02, Brian Gaff wrote:
Well, cannot see your circuit, but from others comments surely if you wanted
it to trigger at a temperature you would need it to suddenly come on at full
brightness not just come on slowly as the bias is altered.
I'd have expected some feedback in the system to make it happen like when
you need to make a relay go on hard.Schmidt Trigger or an IC that does
similar job.


The circuit relies on transistor gain to make the LED transition from
off to on as fast as possible but is not 'triggered' as such.

There is no feedback, just a very simple circuit that compares the
Base-Emitter threshold with a potential divider made from a thermistor
and variable resistor.

On 11/11/17 16:04, Dave Liquorice wrote:
On Sat, 11 Nov 2017 07:26:44 +0000, The Natural Philosopher wrote:

It is a circuit taht shouyld be thrown away though. Ther is no negative
feedback to compensate for tolerance, whatsoever.

The biggest snag is that unless the temperature changes rapidly the
will be quite a band where the transistor is operating linearly and
the LED at variable brightness. Don't you need positive feed back to
make it switch cleanly from one state to the other? You then need to
add some hysterisis to suppress any tendancy ot oscilate just at the
switching point.

Recntly been round that loop with a circuit to swich a bank of IR
LEDs on when it gets dark and off when it's light. Ended up with
something similar, two transistors in darlington arrangement,
LDR/resistor potential divider on T1 base, 10K bias on the T1
collector T2 base connection, emmiters commoned then grounded via a
3R3 resistor, LED and current limit resistor and relay coil between
T2 collector to +V. Certain amonut of "adjust on test" value
selection to get the on and off points set. B-)

schmitt trigger with two transistors is probably optimal.

absolultely defined switchover points.


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"Think about Gay Marriage."