In order to use a sensor (MQ-7: https://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-7.pdf) i need to cycle between 5V and 1.4V. Any ideas on how to do this?

Thank you!

  • \$\begingroup\$ Is the problem you're having controlling a transistor from an Arduino, or generating the 1.4V to switch? \$\endgroup\$ – Pete Kirkham Jan 25 '15 at 22:09
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    \$\begingroup\$ The following answer would probably help (with resistor changes), especially as though I'm not sure that all Arduino regulators could deliver the 150mA required for the heater so you might want an external regulator anyway: electronics.stackexchange.com/a/36879/17064 \$\endgroup\$ – PeterJ Jan 26 '15 at 4:39
  • \$\begingroup\$ Pete Kirkham .. the issue is generating the 1.4V to switch. \$\endgroup\$ – Sorin Trimbitas Jan 26 '15 at 8:32
  • \$\begingroup\$ PeterJ .. this seems to be the exact thing that I need. Can you please help me with the right resistors? Thank you! .. I tried using reuk.co.uk/LM317-Voltage-Calculator.htm but for 1.4 V .. it seems impossible. \$\endgroup\$ – Sorin Trimbitas Jan 26 '15 at 8:32

You probably don't want to use a LM317 as you're close to the reference voltage, which is between 1.20V and 1.30V, so the error in your output could (1.4/1.25)*0.05V = 0.056V, which together with a 5% resistor divider error (0.07V) could put you outside the +/-0.1V tolerance on the 1.4V.

1.4V/33R = 42mA, so there are many fixed LDO regulators which will take 5V ( either from arduino or another fixed regulator ) and output 1.4V. Some have an enable pin as mentioned in this LD6836 datasheet which either enables 1.4V or sets the output to high impedance, so switching is easier than with three-terminal regulator.

  • \$\begingroup\$ Thank you! Will it work ok with a 1.35V fixed LDO like TPS709135DBVR ? (I'm asking because I can get a sample for it). So to cycle power all I would have to do is just send (or not send) a signal to the EN pin. That's really nice! \$\endgroup\$ – Sorin Trimbitas Jan 27 '15 at 7:03
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    \$\begingroup\$ @SorinTrimbitas a quick skim of the datasheet it looks like disabling the output does not put it into a high impedance state, so you'd probably need an external fet on it as well. \$\endgroup\$ – Pete Kirkham Jan 27 '15 at 10:40

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