A yellow 'Pulse' light indicates that there is an excessively large resistance between the positive and negative stimulation electrodes, and the Stimulus Isolator is not able to develop enough compliance voltage to deliver the desired stimulus current. The large resistance could be due to not having sufficient sweat or other electrolyte solutions on the skin, an issue known as Electrode Drying. To solve this, apply some electrode gel to the stimulus electrodes and/or stimulation site to decrease the skin's resistance. The pulse light should now turn green, and you will receive a proper stimulus pulse.
If you require further technical assistance with this or any other issue, please contact ADInstruments Technical Support.