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  1. (*steve*)

    Need help in designing a simple device

    A simple solution will use a pair of rail to rail op-amps designed to operate from a single ended 5V supply, and voltage references/gain set to produce the output you require. This document (from page 19 onwards) gives a pretty good explanation. You will need to do the math to determine the...
  2. (*steve*)

    Light therapy device

    I would recommend you design a circuit board that is single sided and mounts directly to a heatsink about the same size as the board. You'll need several holes in the board to attach it to the heatsink, probably 4 in the corners and a few along the centreline. (oh, and additionally those for...
  3. (*steve*)

    Wiring electronic ammeter / voltage meter

    No, the current must flow through the ammeter. Before you connect anything up, it might be worth using a multimeter to ensure there is no significant continuity between either of the ammeter whites and either of the power supply wires.
  4. (*steve*)

    Wiring electronic ammeter / voltage meter

    Power supply+ and power supply- go to your the output of the charger. Measure+ could go to either your battery +ve (to measure battery voltage) or maybe to the +ve of the solar panels (to measure the solar panel voltage -- not really useful). The current+ and current- go between the charger...
  5. (*steve*)

    Voltage across LEDs in series

    12V assuming the measurement is taken with respect to the negative terminal of the battery. If you were measuring with respect to the cathode of D1 then that voltage might be quite reasonable.
  6. (*steve*)

    R/2R resistor ladder

    For parts 1 and 2, just calculate resistors in series and parallel to simplify the circuit. My recommendation is to use superposition to solve part 3. You have the correct answers, but feeding one potential divider into another does not give the result you describe. Give the resistors...
  7. (*steve*)

    R/2R resistor ladder

    As far as I can tell, the output voltage (expressed as a fraction of Vin) is independent of Vin and R. This one can be confirmed by observation: If you ground A and B and put some voltage Vin on C, you have a voltage divider with 2R above the junction and R below it. The output is 1/3 Vin...
  8. (*steve*)

    Light therapy device

    Sorry about the delay. Here is a BD679 https://www.digikey.com/product-detail/en/stmicroelectronics/BD679/497-5776-ND/1300084 It's in a plastic package so you can screw it through the board and onto the heatsink without worrying about shorting stuff out.
  9. (*steve*)

    Link to personal blog in signature?

    Each of the moderators probably have guidelines which are at slight variance. My general rule is that if the link is in direct answer to a question, directly answers the question, and is something that a disinterested third party might post, I'm OK with it. However, if someone were to answer a...
  10. (*steve*)

    Capacitor type comparison

    Please provide links to the datasheets you have found and at least one image of one of the unknown caps.
  11. (*steve*)

    problem with desoldered buttons

    Yeah, for those switches shown as connecting directly to the pio, I would try adding a 10k pull down to ground. See if that fixes the behaviour without adding any additional strangeness.
  12. (*steve*)

    Newcomer to handling MOSFETS....

    I would recommend being careful. Once soldered into a properly designed circuit you should be pretty safe, however prior to that, static charges can puncture the gate insulating layer. As long as you are working in an area without significant static, using a soldering iron with a grounded tip...
  13. (*steve*)

    indicate where should be NPN and PNP transistors?

    Emitter on top was not what I considered weird. How about all the other permutations? e.g. A (wrong) common emitter. The output remains at approx 0.6V no matter what the input is (within the range of the supply rails as shown). Replacing this with an NPN transistor gives you a more...
  14. (*steve*)

    indicate where should be NPN and PNP transistors?

    That also implies an answer where the transistor was placed completely weirdly would also be correct if one could predict the output. I'm not really comfortable with that -- 12 possible correct answers for each part?
  15. (*steve*)

    problem with desoldered buttons

    It is possible that the long wires are acting as antennas, picking up stuff that the module assumes are button presses. You may be able to fix this with pull ups or pull downs, but it is hard to say which is required and on which wire without investigating the board in some detail.
  16. (*steve*)

    What is ohms law (was Ic=C x dv/dt)

    And area = 1/2 base x height only works for a triangle with straight edges.
  17. (*steve*)

    Ic=C x dv/dt

    Some very interesting (but somewhat extraneous to the topic) discussion about ohms law has been moved to here. The first part of the problem is to get V stated in terms of t. V is in volts, t is in seconds. The general form of the equation is: V = a cos(b.t + c) a, b, and c may be shown as...
  18. (*steve*)

    indicate where should be NPN and PNP transistors?

    I agree that the solution given seems to answer the question, but what eliminates an answer where NPN & PNP are reversed (with appropriate connections and output)?
  19. (*steve*)

    Plotter Cutter Help

    Yeah try it across gnd & 28V, and gnd & -8V. See what happens to both supply rails each time. Sometimes you can tell a component is good or bad with it in circuit. Sometimes the readings are inconclusive and it helps to take readings out of circuit.
  20. (*steve*)

    indicate where should be NPN and PNP transistors?

    Well, they could be either PNP or NPN in those circuits. However, if you know what the transfer function looks like, you can determine which one it is. But hey, I was only considering common emitter and emitter follower (common collector). It's possible that a common base configuration is...
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