diff --git a/bbn_m5atomS3_lite_alternator_freq_to_RPM/README.md b/bbn_m5atomS3_lite_alternator_freq_to_RPM/README.md
index 7a721be..0df6039 100644
--- a/bbn_m5atomS3_lite_alternator_freq_to_RPM/README.md
+++ b/bbn_m5atomS3_lite_alternator_freq_to_RPM/README.md
@@ -11,3 +11,10 @@ The circuit of the boars looks like this:
 
 
 ![PC817 optocoupler isolation module board](img/PC817_optocoupler_circuit.png)
+
+
+There are different variations of the board for 12V input signal or 24V with different pull down resistor on the board.
+
+Select appropriate for your alternator voltage. 
+
+Alternator has a terminal marked 'W' which output a sine wave form of the alternator output.