Search results

Search Forums New Posts
  1. L

    Switching circuit

    Depending on the comparator you select and what the output stage looks like, you might not need the transistor at all to drive the LED. But the transistor would have no function in sensing the voltage as is shown now.
  2. L

    LED ruins logic levels!..

    The voltage across the LED is never going to exceed 2.5 V. Just think of the LED as a forward-biased diode where the forward voltage is 2.5 V instead of the usual 0.7 V. I would replace that diode with a current limiting resistor, and route the input of the gate directly to the output of the...
  3. L

    Switching circuit

    Since you want the voltage drop across R4 to be no more than 200 mV at a current of 0.5 A, you will need to sense when the R4 voltage is just 40 mV at a current of 0.1 A. That seems to be a job for a low voltage comparator, or op-amp run as a comparator, powered from the 5 volt supply. The...
  4. L

    help in 555 timer

    The schematic seems to be the standard configuration for monostable operation, so I would suspect mis-wiring or a bad component. Does the operation change if you substitute a different 555? If you change the 47K resistor to 470K and measure the threshold voltage on pin 6, do you see the...
  5. L

    transistor switch?

    Is it possible the port driver is one of those low power Schottky tri-state outputs such as with the 74LS374? That 2.5 ma @ 5 volt specification may mean that the power supply voltage is 5 volts. The VOH rating is only 2.4 volts at the IOH of 2.5 ma. Transistors such as the 2N2222A have a...
  6. L

    Help with an odd vintage update project...

    My first impression is that this is more a mechanical design project than electrical. There are ways to transfer rotational force between control shafts that do not line up exactly such as flexible extensions, gear drive, and miniature chain pulleys. It would be a touch of finesse to arrange...
  7. L

    Power supply: university project

    One thing that has not been discussed, yet should probably be the first that is considered, is the ripple voltage on the capacitor. This is mostly dependent on the AC frequency, the capacitance value, and the DC current drain. Before designing any electronic regulation, just model the load as...
  8. L

    Battery Supply question.

    The simplest solution would be to try operating it from 12 volts and see if the timing changes. For instance, RC time constants don't necessarily change with supply voltage. The next simplest solution might be to insert a voltage dropping diode coming from the battery, e.g., a Vishay 1N5225B...
  9. L

    Help in understanding the math behind the schematic.

    You seem to have it under control. There is nothing more I can suggest.
  10. L

    Help in understanding the math behind the schematic.

    You are on the right track; however, your placement of the photocell will give maximum collector current when the photocell is illuminated. I thought the project specs called for maximum collector current when the photocell is dark. If I were to mention measuring the Thevenin voltage of the...
  11. L

    Help in understanding the math behind the schematic.

    I have never seen a datasheet that shows parametric curves for a diode that is forward biased going out to 9 volts. It is just not done since it would lead to a massive amount of forward current flow. The base-emitter junction of a transistor acts like a forward biased diode. Your modeling...
  12. L

    Help in understanding the math behind the schematic.

    The referenced schematic is drawn in an unconventional manner which makes it difficult to understand. It appears that the transistor base-emitter junction is forward biased across the power supply. Can't see how this will work, and the transistor will burn out shortly after the power is turned on.
View older results
Top