I generally understand electronics but just do not know what element(s) to use in this case:

There is a little 555 circuit (oscillator) running from just 3 Volt supply and I need to detect when the supply current reaches certain level.
I inserted 1 kOhm resistor between the ground pin (1) and the supply ground and as the oscillator changes its frequency the voltage drop across this resistor goes from 0.1 V to 0.8 V.
I need level of around 0.4 V to trigger another circuit but the detection should not affect the measured voltage significantly, i.e. it should not draw more then microamps.

The circuit triggered by the 0.4 Volt threshold will be running at 9 or 12 Volts.

I feel I should use a FET device but do not have any experience with these transistors, or perhaps an op-amp?

Could anyone help by describing a practical, simple circuit - what voltages at gate, drain and source, or ???

BTW: I experimented with a LED instead of the 1 kOhm resistor and it lights brighter as the voltage rises and I could follow this using some phototransisor, even try to set detection point by inserting a optical obstruction but there must be a more elegant way -
Thank you for your attention.
Vladek

 

Originally posted by Vladek@Apr 26 2006, 09:26 AM
I generally understand electronics but just do not know what element(s) to use in this case:

There is a little 555 circuit (oscillator) running from just 3 Volt supply and I need to detect when the supply current reaches certain level.
I inserted 1 kOhm resistor between the ground pin (1) and the supply ground and as the oscillator changes its frequency the voltage drop across this resistor goes from 0.1 V to 0.8 V.
I need level of around 0.4 V to trigger another circuit but the detection should not affect the measured voltage significantly, i.e. it should not draw more then microamps.

The circuit triggered by the 0.4 Volt threshold will be running at 9 or 12 Volts.

I feel I should use a FET device but do not have any experience with these transistors, or perhaps an op-amp?

Could anyone help by describing a practical, simple circuit - what voltages at gate, drain and source, or ???

BTW: I experimented with a LED instead of the 1 kOhm resistor and it lights brighter as the voltage rises and I could follow this using some phototransisor, even try to set detection point by inserting a optical obstruction but there must be a more elegant way -
Thank you for your attention.
Vladek

[post=16537]Quoted post[/post]​

First off I wouldn't try the LED and phototransistor method. Any change in ambient light will cause a change in the O/P of the phototransistor.

Now for the rest of it. I am not sure where you have placed this resistor and what supply current you are talking about. Can you provide a schematic of your design to help us work this out for you.

 

An op-amp comparator should do the trick for you: http://www.allaboutcircuits.com/vol_3/chpt_8/index.html

 

hi,

as what "thingmaker3" has suggested it will and i might add another 555 will also do it for you.

by the way at what frequency is your oscillator operating?

moz

 

Originally posted by Vladek@Apr 26 2006, 01:26 AM
I generally understand electronics but just do not know what element(s) to use in this case:

There is a little 555 circuit (oscillator) running from just 3 Volt supply and I need to detect when the supply current reaches certain level.
I inserted 1 kOhm resistor between the ground pin (1) and the supply ground and as the oscillator changes its frequency the voltage drop across this resistor goes from 0.1 V to 0.8 V.
I need level of around 0.4 V to trigger another circuit but the detection should not affect the measured voltage significantly, i.e. it should not draw more then microamps.

The circuit triggered by the 0.4 Volt threshold will be running at 9 or 12 Volts.

I feel I should use a FET device but do not have any experience with these transistors, or perhaps an op-amp?

Could anyone help by describing a practical, simple circuit - what voltages at gate, drain and source, or ???

BTW: I experimented with a LED instead of the 1 kOhm resistor and it lights brighter as the voltage rises and I could follow this using some phototransisor, even try to set detection point by inserting a optical obstruction but there must be a more elegant way -
Thank you for your attention.
Vladek

[post=16537]Quoted post[/post]​

 

[post=16557]Quoted post[/post]​

An op amp would be perfect as it has very high I/P impedance.

 

Here in the attachment is the solution using a comparator

 

Originally posted by dragan733@Apr 28 2006, 08:40 PM
Here in the attachment is the solution using a comparator

[post=16680]Quoted post[/post]​

Hi Dragan773,
I just looked at your solution and I just wanted to ask about the purpose of R4 at the output of Opamp.. is it like a pull up or pull down resistor ? if so , please explain in detail the reason for it.

Thanks

 

Originally posted by Mazaag@Apr 29 2006, 02:51 AM
Hi Dragan773,
I just looked at your solution and I just wanted to ask about the purpose of R4 at the output of Opamp.. is it like a pull up or pull down resistor ? if so , please explain in detail the reason for it.

Thanks

[post=16681]Quoted post[/post]​

Hi mazaaq,
because all comparators are with open colector, therefore the output resistor R4 goes to +supply.
I did a change in the schematic, see it again

 

Originally posted by dragan733@Apr 28 2006, 09:06 PM
Hi mazaaq,
because all comparators are with open colector, therefore the output resistor R4 goes to +supply.
I did a change in the schematic, see it again

[post=16682]Quoted post[/post]​

Hi Dragan..

thanks for your reply..but i'm not quite sure what you mean...
could you please explain it in more detail ?
P.s: i'm a noob.

thanks

 

Mazzag,

Look at the attached diagram from the LM193 Application Note.

You will see the output of the comparator has an open collector.