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  1. R

    pull down resistor for npn relay driver

    Yes, I have a diode across the relay coil.
  2. R

    detecting low voltage with PIC

    Thanks. Do I need to add any additional circuitry to the PIC that will be monitoring the LED voltage at its cathode? Or can it just be a straight connection?
  3. R

    pull down resistor for npn relay driver

    Hi Yes, i was having trouble uploading the schematic. Yes i have a 15K from the PI I/0 line to the base of the NPN. It is in common emitter mode with no emitter resistor, I was thinking of around 56K to 68K. Is there anything that determines the value?
  4. R

    detecting low voltage with PIC

    Hi, I have a Pic micro that drives a LED. The circuit is 3V3 through a 270R resistor, to the anode if the LED and the cathode is connected to a I/O line of the PIC. So a logic 0 lights the LED, a logic 1 keeps it off. Can I connect this cathode / PIC IO line to another PIC microcontroller input...
  5. R

    pull down resistor for npn relay driver

    Hi all, I have the attached circuit diagram. I wanted to know if there is any way of calculating the value of the pull down resistor. I don't want the relay to be driven on when the microcontroller is switched on. My understanding was that if the output from the PIC is OV. Then the transistor is...
  6. R

    Clarification on LED millicandela and light output

    Yes it is. There are 4 in total to give the required brightness. I just found that out today. Thanks for all the help. How should I compare the outputs, I was thinking of mounting them on the ceiling in a dark room. Then visually checking that there are no big discrepancies in the output light...
  7. R

    Clarification on LED millicandela and light output

    Thanks all for your replies, much appreciated. Unfortunately i can't add multiple LEDs the second LED is supposed to be a drop in replacement (+ resistor change) for the first one.
  8. R

    Clarification on LED millicandela and light output

    yes, i am after a visual match. the LEDs are used in an escape light for an alarm unit. So will light up the exit point to evacuate the building. They have different viewing angles. So it is best to power what i have at the moment i.e. LED1. Then use LED2 in its place with a POT until I get the...
  9. R

    Clarification on LED millicandela and light output

    Hi all, I have two LEDs which have the following specifications LED1 LED2 VF 3.0V 3.2V IF...
  10. R

    Detecting positive and negative edges

    Thanks i will give this a try.
  11. R

    LED replacement

    Hi, I only have the printed sheets not in electronic format. This is the information. The primary LED (used at the moment) has the following parameters: Luminous Intensity(mcd) = 3500 (min) 4500 (max) IF (forward current) = 30mA VF (Forward Voltage) = Min (2.7V) Typ (3.0V) Max (3.6V)...
  12. R

    LED replacement

    Hi, I am looking for a secondary source for LEDs in a project. I am looking at the following specifications to make a comparison, have missed anything out? Dimensions Pitch Colour VF forward voltage If forward current Reverse current Min and Max current Lumens output in MC milli candela...
  13. R

    Low value resistors

    Is a 4 wire system achievable using FETs??
  14. R

    Low value resistors

    Ok. The resistance values are inserted in series with a 3V supply to alarm units. This way we can introduce high impedance battery faults into alarm units.
  15. R

    Low value resistors

    No, the software just selects the value required, switches the relay and that's it. So I need help with the hardware side. It's proving tricky.
  16. R

    Low value resistors

    Hi, I have a application in which i am switching low value resistors i. e. 0.1R, 0.2R, 0.4R etc to a unit under test. I have come across a problem, track resistance and cable resistance which override the resistor values. These are software selectable so need relays to switch them to the end...
  17. R

    LED monitoring

    Would I need to use a interrupt on change pin of the PIC?
  18. R

    LED monitoring

    No I don't need to see if the LED is illuminated. But need to see it has been activated.
  19. R

    LED monitoring

    Hi all, I have a project in which I need to monitor a couple of LEDs for activity. This is part of an automated system. The LEDs are driven from a microcontroller. 3.3V, then a resistor with the LED cathode connected to the micro I/O pin. So a low signal will trigger the LED on. Could I...
  20. R

    Detecting positive and negative edges

    Yes, I can do that. I will post some screenshots asap. Thanks for your help.
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