RFI suppression in sensor wire
RFI suppression in sensor wire
(OP)
The following circuit is an input section of a break wire alarm. The sensor is a 20m long wire which activates the alarm when broken. The wire keeps the logic input low and when it's open the logic input goes high. The first priority in designing this circuit was robustness against false alarm due to RFI induced noise and low power consumption as it's powered by a 9V battery. Although the protection seems to be enough I'd like your input. Additionally, other suggestions regarding the design of PCB will be welcomed.





RE: RFI suppression in sensor wire
For cost, you could replace your TVS diode with another 1N4148, and maybe increase R11 and R12 to 100 Ohms.
RE: RFI suppression in sensor wire
You could add three bypass capacitors, across the wires plus two to ground, to the wire-side circuit before the diode D1, where any RF EMI would still be RF. After the (detector) diode D1, the RF EMI is effectively 'detected' to DC making rejection that much more difficult.
You could also add more capacitance at the gate input, to slow the response to the longest acceptable limit (perhaps 1 second?) to provide some rejection of short pulses such as neighbourhood lightning or human ESD.
All this depends on how critical is the application.
RE: RFI suppression in sensor wire
Kindest regards
D
RE: RFI suppression in sensor wire
Z
RE: RFI suppression in sensor wire
In my opinion, capacitors of 1nF or less should be SMT with short trace runs, otherwise lead inductance largely negates the presence of the capacitor. Larger caps can be leaded since with their larger values they affect lower frequencies, and lead inductance is less an issue.
RE: RFI suppression in sensor wire
RE: RFI suppression in sensor wire
"Trip wire" circuits went out of use in war zones ans industrially 50 years ago.
Optical or other types of proximity detectors are far more reliable.