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  1. duke37

    Relay Problem

    Look up Wheatstone bridge (Wikipedia)
  2. duke37

    Need help with Heathkit io-10 o-scope repair

    Without the rectifiers, the transformer will only be supplying the heaters so it should run with minimal warming. It will be a big limp and have a long time constant so leave it for several minutes. and yes, nothing should happen. You coud put a 100 ohm resistor in the cathode of the 6X5 to...
  3. duke37

    Relay Problem

    The gap between on and off can be reduced by increasing the feedback resistor but 1M is about as high as you should go with home made construcion because of possible stray leakage. The hysteresis can also be reduced by reducing the two resistors in the bridge but this will increase the power...
  4. duke37

    Relay Problem

    OK, this is what I suggest. Please tell us how you think the various parts of the circuit works. There is little point in copying without understanding. Start with the bridge, what does the 741 do, why put a base resistance in, why connect the transistor this way. How does the feedback work...
  5. duke37

    Relay Problem

    That is rubbish.
  6. duke37

    Need help with Heathkit io-10 o-scope repair

    You are starting with a dodgy project. The scope has very high and dangerous voltages. The problem with the 6X5 is probably due to leaky electrolytic capacitors which have 'gone off'. I would replace rather than reform. The sparking at the high voltage rectifier could be due to a leaky...
  7. duke37

    Relay Problem

    You could look up the data to save us the trouble. The relay data you refer to has a host of different coils so it is not clear what the requirements are. Assuming the coil is for 12V and run on 12V, the current will be 37.5mA. With a bit of voltage drop across the transistor it would be OK to...
  8. duke37

    Relay Problem

    I have never met circuit wizard. The relay driver circuit is not the usual one. Usually, the transistor emitter is connected to ground. A resistor is added to control the base current from the 741. The relay is connected between collector and positive supply. A diode must be placed across the...
  9. duke37

    Relay Problem

    I do not see how this will work, You need to connect the thermistor and potentiometer as in your original circuit.
  10. duke37

    Ring Modulator (dalek emulator)

    Here is a low frequency phase shift oscillator from 'Transistors Theory and Circuits' byK.J.Dean 1964 This runs at 11Hz with 0.1µF capacitors. It uses pnp in a Darlington configuation. I would use an npn Darlington (MPSA13) with reversed supply.
  11. duke37

    15 min alternating circuit

    Depending what you mean by switching between positive and negative wires. If the power is coming from the batteries, then the ground (-) and positive will need to be interchanged and there will not be a common ground on the output.
  12. duke37

    15 min alternating circuit

    A CMOS 4093 can be made into an oscillator and will run on 3V but you will need a very good capacitor and good mounting of components to get a reliable 15 minutes. A 4060 contains an oscillator and divider which would be better, running at 18Hz? The easy way of alternating the voltage would...
  13. duke37

    Relay Problem

    You will need two resistors in series from the 12V to ground. The junction should be connected to the + terminal. A couple of 10k resistors would probably do. The junction would be at 6V. The voltage of the junction of the sensing circuit will be above or below 6V so the amplifier will switch...
  14. duke37

    Wireless Headphones

    What is the red light supposed to show? If it is to show that charging is taking place, then either the batteries will not accept the charge or the cable is not making a connection. I suggest you measure the output voltage of the charger.
  15. duke37

    Permanent Magnet Alternator

    Normally he alternator will have the magnet in the rotor so there will be a vaying flux in the pole pieces in the stator. The voltage produced will depend on the rate of change of flux which will be dependant on the rotor speed and the number of turns on the pole pieces. A lot of turns gives a...
  16. duke37

    Old lamp from the 70's

    I am not familiar with 120V fluorescent lamps but if the circuit is the same as for 230V, the capacitor is just to provide a good power factor and the lamp will work without it. The resistor is to discharge the capacitor when the lamp is turned off for safety. The choke that you have pictured...
  17. duke37

    Adjustable Voltage Regulator

    There is a difference from the diagram. The diagram shows R2 as 240 ohm, you have used 510 ohm. This may not be important but high value resistance here is reputed to lead to instability. To make the potentiometer less sensitive, use a lower value.
  18. duke37

    Wireless Headphones

    Seems as if the battery is dead. Can you measure the battery voltage? Snap
  19. duke37

    Using a voltage converter...

    The Amazon reference is to a mirror. There are two differences in the power, voltage in europe will be 230V not 110V. Frequency will be 50Hz not 60Hz. Depending on the equipment one or both of these differences will be important. I have just managed two things working which were brought into...
  20. duke37

    Adjustable Voltage Regulator

    Presumably the 220 ohm resistor is connected between the output and the sense terminal. These differ by 1.21V so the current will be 5.5mA. To increase the voltage output, a resistor has to be connected from the sense terminal to ground which passes 5.5mA so to get an exra 2V you will need...
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