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My circuit table schematic

I wanted to control speed of my four coreless DC motors using Arduino Nano. Then I realised my 3.7V 380mAh 25C and 720mAh (110mAh on picture is just to hold my battery's place) 25C LiPo batteries won't meet the voltage requirement of a Nano. Then I came up with the idea of using Pro Mini instead of Nano; however I couldn't find a 3.3V pin on Pro Mini to feed NRF24. On Nano, I was using a 3.3V pin and 10uF capacitors between Vin and Gnd legs of NRF24.

So my question is: can I feed NRF24 directly from Vcc pin? Or as an alternative what would happen if I fed NRF24 directly from battery and reduce 3.7V to 3.3V via another TIP122 transistor?

Note: I am using 4 of TIP122 transistors whose gate legs are connected to Pro Mini's 5, 6, 9 and 10 pwm pins (I forgot to add base-resistors). Do you also think it is a reliable way of controlling the motors' speed?

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can I feed NRF24 directly from Vcc pin?

NO: the pro mini is either 5V or not strong enough to power the RF24. EDIT: the 3V3 pro mini "could" power the RF24 via VCC, if you use the cap and if it's not the PA&LNA model (or in full power mode). Best practise is to use an external regulator.

if I fed NRF24 directly from battery and reduce 3.7V to 3.3V via another TIP122 transistor?

NO: lipos are 4.2V when fully charged, and they can be used until 3V (I mean 3V sag voltage, when the load is powered, not when disconnected): External 3V3 regulators is the way to go.

Do you also think it is a reliable way of controlling the motors' speed?

These are PWM output, so yes, why not? Also, add flyback/freewheeling diodes (and -maybe- 100nF polyester caps should help) between the motors terminals.

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  • Please consider that since the OP said "switch from nano to pro mini" he will be using the arduino board powered from a 3-4V source, rather than 5V. This means that connecting to the Vcc pin is ok, provided that the voltage is not greater than 3.6V (and as you explained LiPos go even higher than 4V). So if the pro mini is powered by a linerar/switching regulator to 3.3V, then Vcc can be used
    – frarugi87
    Jun 20, 2019 at 11:53
  • @frarugi87: the pro mini only has 150mA MAX on VCC, as i said it's not a matter of voltage but current instead (assuming a 3V3 pro mini is used). The capacitor helps but stable performance can't be guaranteed with the VCC supply, especially with other devices connected to the pro mini. forum.mysensors.org/topic/3961/…
    – theGarz
    Jun 20, 2019 at 12:17
  • your comment made me think that probably I misunderstood the OP intentions. For me, the best way to power the pro mini is to directly connect the Vcc to the battery input, bypassing the onboard regulator in the pro mini. But probably yes, the OP wanted to connect the battery to the RAW pin, and so the reply is "it depends". As long as a standard NRF24L01+ is used the current generated is ok, but if a high power board (NRF24L01+PA-LN) is used then the regulator is not enough
    – frarugi87
    Jun 20, 2019 at 12:25
  • Connecting the Pro Mini straight to the power bus alongside motors can cause noise-related issues and sudden current draw due to motors being turned on can cause the MCU to reset. Some voltage stabilization is required, in addition to that, the nRF24L01+ is suspectible to noise, I can't imagine it working in such a setup without at least one mF of capacitance (half electrolytic and half ceramic to get rid of HF as well) attached to it.
    – Avamander
    Jun 20, 2019 at 17:33

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