The packet states NRF24L01. I purchased ASHATA NRF24L01+ Breakout Adapter as it has a voltage regulator down to 3.3 and a capacitor to smooth out the input power. Running code in the loop which transmits every second and then get a response from the receiver works all day long. To take button bounce out of the frame I've done some more experiments. Switch both receiver and transmitter off. Power up the receiver using a transformer. Plug a USB cable into the transmitter. You get a flash response from the receiver and the acknowledgement LED flashes for a second. Pull the USB cable out of the transmitter, count to 5 and plug it back in again. The transmitter acknowledgement LED lights for a second but the receiver does not flash a response. After the first operation, if the transmitter is switched off, the receiver will acknowledge the transmission but not process the instruction. Radio.available returns false. I'm wondering if I should clear the receive buffer with flush_rx.