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  1. KrisBlueNZ

    Help in feed input of sensor with PNP output to controller (pic16f877a)

    What's the sensor's part number? Manufacturer? Where did you get it? The diagrams you've uploaded are block diagrams. If you need more specific help you may need to provide a more specific diagram, i.e. a schematic.
  2. KrisBlueNZ

    Simple yet confused

    I think the reason I suggested separate switches is that if the charger needs to be disconnected for a short time during the changeover, so it will detect that a new battery has been connected, you could do that using a switch with a centre OFF position, but if you use the same switch to select...
  3. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    If you want to be able to provide up to 4.0V into those motors from a 4.5V power source, the minimum voltage drop you can achieve in the emitter follower is the important factor. That's why you suggested using a germanium transistor, and that's why I suggested a negative low-dropout regulator as...
  4. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    Well, the obvious solution is to power the unit from four 1.5V cells instead of three. Another solution would be to use a TI TPS72301 low-dropout negative regulator, which is available from Farnell UK...
  5. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    Yes, right. That's interesting about the ): getting included in the URL. Must be a bug in the forum software. It parses URLs automatically and usually gets them right.
  6. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    Well, that's an interesting idea! Yes, germanium transistors do have lower Vbe than silicon transistors. They aren't very common nowadays, but NTE (http://www.nteinc.com) make them. You can also get them through eBay, but buying electronic components through eBay is a bit like Forrest Gump's...
  7. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    Yes. Put a 0.1 µF ceramic across pins 4 and 8 of the dual op-amp as close to the IC as possible, and 10~100 µF electrolytic across the rails somewhere else. The emitter follower will drop a minimum of about about 0.7~0.8V so you won't get quite 4.0V from a 4.5V supply. Because of the voltage...
  8. KrisBlueNZ

    Changing a motor's direction each time power is supplied

    I don't know whether this will be useful to you or not, but I designed a circuit to do what you want - to prove to myself that I could. My priority was low cost. I was aiming for simplicity as well. I'm not sure whether I achieved that, but I can't see any way to simplify it further without...
  9. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    The OPA2350 looks good. It has a much lower input offset voltage, which isn't really important, and it's much faster, which also isn't important. It draws a lot more supply current, but 15 mA isn't a big deal compared to the current the motors could be drawing. The output swing is good.
  10. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    TLV272 is available in DIP and is the cheapest suitable device. http://www.ti.com/product/tlv272 Unless you intend to mass produce your device you should not be asking for free samples from TI. Digikey have the TLV272 for USD 1.21...
  11. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    The dual op-amps I've suggested are 8-pin devices. You can use one op-amp as the Schmitt trigger for the oscillator and the other as the inverter to drive the second transistor.
  12. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    That CD40107B has open-drain outputs, and it doesn't have Schmitt trigger inputs. I don't know what you would want to use it for. Are you reading my posts fully? Try re-reading them carefully.
  13. KrisBlueNZ

    Simple yet confused

    Another approach would be two separate DPDT switches, one to select which battery is connected to the charger (this one would have a centre off position: http://www.digikey.com/product-detail/en/100DP3T1B1M1QEH/EG2413-ND) and a second one to select which battery is connected to the load (either...
  14. KrisBlueNZ

    Eye Movement Desensitization Rehabilitation device

    A "simple inverter" will not make an oscillator the way a Schmitt trigger will. You could use a single-gate Schmitt trigger inverter: http://www.digikey.com/product-detail/en/TC7SZ14FU(T5L,JF,T)/TC7SZ14FU(T5LJFT)CT-ND but these are only available in SMT packages and you can't exceed 5.5V...
  15. KrisBlueNZ

    mosfet circuit?

    Have a look at https://en.wikipedia.org/wiki/Servo_control and tell us whether the servo control signal you have matches the description. According to that article, the servo is controlled according to the pulse width; a 1.5 ms pulse is "centre position", and a pulse width range of typically...
  16. KrisBlueNZ

    Simple yet confused

    Do you know how the charger will handle being disconnected from one battery and immediately connected to a different battery? NiMH batteries are charged through a series of phases - pre-charge, then main charge, then top-off charge, at least. You might have to switch the charger off and on, or...
  17. KrisBlueNZ

    mosfet circuit?

    One small point: MOSFETs are easily damaged by high gate voltage and static electricity. You can protect them with a zener diode with its cathode to the gate and its anode to the source. The zener diode can be just a 0.5W type. Its zener voltage should be a bit higher than the level of the...
  18. KrisBlueNZ

    Unwanted Comparator Offset

    A comparator output will swing nearly all the way to the rails. Something like an LM311 (single), LM393 (dual) or LM339 (quad). These have open collector outputs and need pullup resistors. Op-amps are designed to be used as amplifiers, where rail-to-rail output swing is often not needed. Some...
  19. KrisBlueNZ

    Help with 4510 bidirectional visitor counter

    That schematic is poorly drawn. I've highlighted some obvious issues I noticed. The path I've marked with red lines may not be wrong, but the way it has been drawn makes me suspect that the diagram hasn't been checked very carefully and that makes me suspect it has other "real" errors. The...
  20. KrisBlueNZ

    2N2222 NPN Transistor

    The standard wire-leaded 2N2222 comes in a TO-18 metal can package. These are unnecessarily expensive and barely used at all nowadays. A much cheaper and equally good package is the TO-92 plastic package (see https://www.google.co.nz/search?q=TO-92&tbm=isch). If you're in North America, a...
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