I have similar problem with the HC-SR505 sensor. First few seconds it holds 0 on the output pin, and then always 1. I think it was bad batch of sensors. Bad batch in chinese manufacturing means thousands (or hundreds of thousands) bad modules flooded the market.

Meanwhile I decided to choose one of options:

1. Stick with HC-SR501 sensor. It's bigger but works more or less.
2. Buy expensive ($5.90) PIR sensor from seeed.
3. Build my own PIR module by my required dimensions and function (in progress).

I have similar problem with the HC-SR505 sensor. First few seconds it holds 0 on the output pin, and then always 1. I think it was bad batch of sensors. Bad batch in chinese manufacturing means thousands (or hundreds of thousands) bad modules flooded the market.

Meanwhile I decided to choose one of options:

1. Stick with HC-SR501 sensor. It's bigger but works more or less.
2. Buy expensive ($5.90) PIR sensor from seeed.
3. Build my own PIR module by my required dimensions and function (in progress).

Update: From 5 sensors received 2 do the job. Supply voltage must be 5V or higher. First 20-60 seconds the output is not reliable and should be ignored. Then it works OK (if it works at all). Since the module has 3.3V LDO onboard the output signal is 3.3V.