I have a control signal to a normally open (NO) SSR and I'd like to verify its frequency. The relay's DC+ terminal is always connected to 5V but the DC- is not always connected to GND. The GND connection is being switched by a GPIO connected to the base of an NPN transistor.
But since my signal is alternating between no connection and a connection, how can I measure the pulse with an oscilloscope?
If I understand your description, there is NO pulse without a complete circuit. No ground,no pulse measured to ground. You can prove that by putting a small value resistor in series between the pin and the SSR and measure the voltage with your scope.
Paul
Hi, @feynman137
Measure the pulse at the INPUT to the SSR, you should see the signal that is on the collector of the NPN transistor with respect to the NPN transistor gnd.
Tom... 
Hi Tom I am a little confused as to what you are suggesting. Could you mark where on the diagram below you suggest to put the oscope pulse probe and GND?
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