Here is the inside of the light reflectance sensor (top) and
a schematic drawing of it's circuitry (bottom). First, carefully remove the
phototransistor as shown. Then, attach the three wires that we will connect
to the heart sensor (show in bright yellow). Hint: if you leave the leads from
the phototransistor when you cut it off, you can attach the two corresponding
wires directly to them, rather than to the circuit board itself.
Thanks to Hiroki Shirakawa
for the photograph and schematic (updated by Michael