Use a 5 volt power supply for the comparators and the 24 volt supply for the input .They must be separate of course but they must share a common ground connction.What supply voltage would you suggest in order to get a 5V TTL?
Agreed. We need to know the output resistance of the sensors and their sensitivity to be able to come up with a reasonable value of feedback resistance. And what's with those 74k resistors? 74k is not a standard 5% or even 1% value.nettron1000 said:Good points Ron, i'd just like to add that the circuit would work better if a bit of hysteresis were added. This can be done by connecting a resistor between the output and the non-inverting input (+). This will give the circuit a snap action to slowly rising inputs and will behave more like a digital output.
The link i provided goes into explaining that.
1. You can save power (and heat) by getting rid of the 130 ohm resistor, and changing the value of the 910 ohm resistor. LEDs are basically current-operated devices, i.e., the voltage across them is doesn't change much as you change the current through them. From the datasheet, determine how much current you want to pass through them and how much voltage will be across them at that current. Subtract that voltage from 24 volts and divide the result by the current. The result is your new resistor value.[email protected] said:Thank you every one for your help, it is greatly appreciated and I'll certainly be taking it on board.
In answer to some questions:
1) The top part of the switch is to switch between two sets of LEDs. Each LED will be labelled and it will tell the user whuch of the pressure sensors has been selected for measuring inlet pressure and which one for outlet pressure. As it actually switches the contacts into the comparator at the same time it removes any confusion that could occur if there were two switches
2) I've used the second comparator because i thought, misguidedly probably, that the output voltage of the comparator would be negative and by passing it through the inverting input I would get a poisitive voltage output.
3) I used the 74k resistors because there were a load in the workshop and as I wasn't worried about current I used them so they would stay cool in the plastic enclosue this thing will eventually go in.