how to have tenant's signature after installing new fire alarms

[Edita D.]

Hey guys,
we purchased a 4-plex a year ago, however the tenants did not sign a lease with us rather than they stayed on a month-to-month basis.
Since the CA law is very strict about smoke alarms, we just installed new fire alarms/carbon monoxide detectors at all units. I created an addendum for tenants for them to be familiar how often they are responsible to check the devise as well as test it and change batteries. However, since there is NO LEASE we can not attach an addendum... How do I have the tenants sign it? If I just send them a notice, in case of fire (due to tenant's negligence to test the fire alarm), there will not be proof we took all preventative steps, since there's no signature.
Suggestions?
Thanks!