Don’t believe the “TTL” part in the description - it isn’t true TTL, it is an amplified and clipped version of the light sensor’s analogue output, and as such it does not have defined output levels. It does work normally, but it can be influenced by ambient light, so your meter needs to be in a shaded location.
There’s a lot written about it here: First try with EmonPi - Pulsecount stuck at 1 - #16 by Robert.Wall and Adjusting sensitivity of Optical Pulse Sensor - #21 by Kempson