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  1. KrisBlueNZ

    Voltage sensor

    I think the best way is to subtract a fixed voltage from the battery supply. A zener diode is not accurate enough, but you can use an adjustable shunt regulator such as the common TL431. Subtracting say 10 volts means the working range of your output voltage would be 2V to 3.7V. If you can set...
  2. KrisBlueNZ

    Optocoupler Output

    As Harald says, exchanging the opto transistor and resistor has no effect if the transistor circuit is driving a high impedance. Since you show a supply voltage of 15V, I assume you are driving a CMOS input from the optocoupler. CMOS inputs have an extremely high impedance and the value of the...
  3. KrisBlueNZ

    Help on circuit

    Yes Fish, I found a couple of amazing low-Vgs MOSFETs and linked to them back in post #9. They are: Alpha & Omega AO4402 (http://www.digikey.com/product-detai...9-1-ND/3621495) USD 1.03 Rds(on) typically 5 milliohms and guaranteed under 7 milliohms at Vgs=3.6V SO-8 same pinout as FDS8449...
  4. KrisBlueNZ

    Help on circuit

    Fish, why use such a large AVR? Also I would use a MOSFET with a lower Vgs specification. That one only has a guaranteed ON-resistance specified at 4.5V and the graph shows that the typical (not worst case) ON-resistance climbs pretty steeply below about 3.5V.
  5. KrisBlueNZ

    Help on circuit

    Good on you Fish :-) I agree, a small micro driving a low-Vgs MOSFET is the best solution. I only didn't suggest it because of the need to program up a device. You can get a perfect PWM control ramp that starts immediately when power is applied, and the whole circuit is nothing more than the...
  6. KrisBlueNZ

    Driving Peltier Device through L298N to control Temperature

    Good idea Fish! A DPDT relay wired as a reversing switch would be simpler and cheaper than an H-bridge.
  7. KrisBlueNZ

    Help on circuit

    I'm not sure whether this 6V supply will be useful or not. It depends on how it relates to the 3.6V output from the PIR device. How does the 6V supply connect to the PIR device? And do you know how the LED control output from the PIR device relates to the 3.6V supply and the 6V supply? For...
  8. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    Yes, BC559C will be fine too.
  9. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    Oops! I listed some wrong transistor part numbers in post #27. You can use 557B, 557C, 558B or 558C. Not 548.
  10. KrisBlueNZ

    Hard Drive PCB repair

    Check the fuses for resistance from one end to the other. A fuse should read zero ohms if it's OK. If you measure any significant resistance (i.e. more than you measure with the probes shorted together), the fuse is blown. Yes, zero ohm resistors should measure short circuit, i.e. zero ohms...
  11. KrisBlueNZ

    Driving Peltier Device through L298N to control Temperature

    This is what I'm suggesting. The main power supply generates a voltage around 30~35V at about 5A maximum. This voltage feeds into a buck converter with variable output voltage or current, which feeds through the H-bridge to the Peltier device. Ideally the buck converter would provide a...
  12. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    Cool :-) You too. You can use any general purpose PNP transistor. If you're in England or Europe, BC557B, 557C, 548B or 548C are more common than the American 2N series. There's a place for your location in your profile and it can help others if you fill it in. Edit: suitable BC5xx...
  13. KrisBlueNZ

    Driving Peltier Device through L298N to control Temperature

    Sure, you can use an H-bridge to switch the polarity of the current. I would not use an L298N for this; use some big grunty N-channel MOSFETs with an H-bridge driver IC. As you said, you should power down the circuit while the polarity is being reversed. The H-bridge would be a separate stage...
  14. KrisBlueNZ

    Is it normal for an opamp to "rectify" a signal?

    Great article Chris! Very clear explanation. Were any changes made as a result of what you saw? Would you recommend that the heavy load should be powered through a separate circuit?
  15. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    Thanks :-) The resistor you have shown as R2 should not be there, and the 27k resistor marked R3 should be connected to the top end of the LM336Z, not in series with the other trimpot. Both trimpots should be connected directly from VCC to 0V. The value of 300 ohms for R6 is only approximate...
  16. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    Sure, you can do that. And that would save current - at least until the battery voltage gets low. You wouldn't be able to see that the circuit was working until the battery became low. I assumed you wanted the three-way indication. If you don't, then a simple low battery indication circuit...
  17. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    OK, here's my suggested circuit. The changes are only in the LED circuit. Some features of this diagram are needed for LTSpice, the circuit simulation software, and aren't meant as changes from your original circuit. R1~4 represent the two trimpots. They're shown as resistors in this diagram...
  18. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    I'm still thinking about a good way to deal with the current consumption issue. The best way would be a switching converter that would boost the current while it reduces the voltage, so the LEDs could run at 10~15 mA but the load on the battery would be less than that. But that would add...
  19. KrisBlueNZ

    Alarm reset timer

    Great! The circuit shouldn't have any connection to mains earth. Were you connecting mains earth through to the negative rail of the circuitry, with the downward-pointing triangle symbol? No! That's not supposed to be connected to mains earth. It's just a "0V" reference symbol for use by...
  20. KrisBlueNZ

    6 CELL LI-ION 22.2V BATTERY LEVEL MONITOR

    My time isn't THAT valuable LOL :-) But yes, you can change the circuit as I described in post #8 so that when the green LED turns ON, the load current on the battery doesn't increase. In fact, it decreases slightly. This is because the LED's drive current, through the 1.5k resistor, is always...
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