Tech Tips: Latching Door Annunciator Circuit

Dec. 1, 2016
This circuit is inexpensive, robust, and easy to troubleshoot.

This month we will take the circuit introduced last month ( ‑ which turned red and green LEDs on and off to indicate the position of a door ‑ and add a latching feature. If the door is opened, the red LED will illuminate and remain illuminated until the circuit is reset, and the LED cannot be reset until the door is closed again.

For example in a healthcare facility, it is desirable to make it necessary for a staff person to go to the door to reset it so a staff member can survey the situation and be certain the door is properly reclosed. It also makes it more likely that if the door is opened while staff is otherwise occupied, there will be evidence the door had been opened in their absence.

Many times if the Authority Having Jurisdiction (AHJ) is calling for door monitoring, they will also be asking for the reset of an alarm occur manually at the point of violation, rather than other alternatives such as resetting from a central location, or the alarm automatically resetting itself after a certain length of time has elapsed.

Numerous products can perform a similar function, but they may involve additional features you do not want, may be digital and contain delicate circuitry, and may add unnecessary cost to your application. This circuit is inexpensive, robust, and easy to troubleshoot.

A key component is the pushbutton.  A large assortment of pushbutton switches are available.

Switch contacts can be ‘momentary’ (such as a door bell button which actuates only while being pressed).

Switch contacts can be ‘latching’ or ‘toggle’ (such as a wall switch to control room lighting). It remains in either one of multiple states. On-Off is a common type, but you will also see three or more position switches which offer different functions. For example a switch can be “ON-OFF-ON”; where the switch can be actuated in two different directions, and control two different circuits.

Switches can be controlled by a lever, a button, a key, or electrically by applying voltage to a coil, and the contacts on the relay “change state.”

Switches can be controlled by a credential reader, keypad or sensor such as a NFC type device.

Where the switch is meant to be a convenience to control something, a lever or pushbutton is fine; but where it is necessary to restrict the control of the switch contacts to avoid abuse or a hazard, then a means of authorization is used.

Authorization is how a device is controlled. Authentication is who is given authorization. A basic example of authorization would be possession of a key operates a lock. With keys and memorized codes, merely having the key or knowing the code is no assurance that the right person is holding the key or entering the code. That is why we use methods to authenticate the holder.

STI Products’ Universal Button (UB-1) Kit is a versatile device I frequently use for custom projects. It’s sturdy, versatile, can be illuminated, and is supplied with a variety of signage for many uses.

That’s why it is called a kit. You get many accessories with the UB-1 product. In particular, is good for resetting a latching door annunciator.

STI also provides its line of “Stopper” products intended to protect switches and other equipment from damage or careless usage, and there are stoppers which work perfectly with the Universal Button Kit.

The Universal Button Kit momentary button helps eliminate having to stock dozens of pushbuttons, and wasting precious time and money tracking down just the right button for a customer. It offers many options to provide just what a customer wants in one convenient package. Button is ready to be assembled out of box and is easily installed within minutes on site. Universal Button offers over 300 combinations for the price of a standard one configuration button. The faceplate is constructed of cast aluminum metal, allowing it to take tough knocks in stride, and is slightly oversized to cover old plate installation marks.

The installer simply has to choose which lens cover, button insert and interchangeable message plate works best for the customer’s need, and easily snap them in place. This allows the button to be mounted in a matter of minutes instead of waiting weeks for a specific button to be ordered and delivered.

Features include:

  • Momentary button offers many options to provide just what a customer wants in one convenient package.
  • 100,000 operations.
  • One year guarantee on electro mechanical and electronic components.
  • UL Listed to U.S. and Canadian safety standards.
  • Temperature: -40° to 185° F (-40° to 85°C).
  • Mounts to standard electrical handi box.
  • Single gang plate slightly oversized to cover old plate installation marks.
  • Illuminated 12/24 volt LED button and illuminated message light kit.
  • Two sets of independent Form “C” contacts. · Contacts rated at 10A, 1/2 HP, 125/250 VAC, 6A, 30V.
  • Five inserts for the button included – EXIT, PUSH, PUSH TO EXIT, handicap symbol and blank.
  • Three different lens covers included – red, green and blue.
  • Ten color coated message inserts in green, blue, yellow and white.
  • Plate messages inscribed with EXIT, EMERGENCY and translucent blanks (allowing for custom labels).

The STI Universal Stopper is an indoor/outdoor low profile or dome polycarbonate cover used to protect devices such as dual action pull stations, keypads for entry systems, intercom stations, emergency buttons, electrical light switches, duplex plugs, etc., without restricting legitimate operation.

The versatile cover offers excellent protection against physical damage (both accidental and intentional), dust and grime as well as severe environments inside and out.

Cover options include Dome or Low Profile, with or without horn, flush mount, surface mount or waterproof back box models. Maximum protrusion is 4 inches deep. Cover has the same footprint as the Mini Stopper® but accommodates most dual action pull stations.


  • ADA Compliant.
  • Three-year guarantee against breakage of polycarbonate in normal use (one year on electro mechanical and electronic components).
  • Top and bottom of waterproof backbox are threaded for 3/4” NPT rigid conduit.
  • Sized to fit most pull stations.
  • High strength continuous hinge.
  • Cover is molded from thick, tough polycarbonate material and UV-stabilized to keep cover clear.
  • Gaskets included.
  • Dome models have optional relay output with 12-24 VDC power input.
  • Dome models available with or without 105 dB horn.


  • Cover available in low profile or dome shape.
  • Flush frame, surface frame or waterproof back box (with sealed or open internal mounting plate), enclosed flush back box.
  • Locking tab (not for use with fire pull stations).
  • Dome models available with red, green, blue, yellow, white or black label hood/horn housing with custom or no label.
  • Available for red units “In Case of Fire...” label.

More Info:

About the Author

Tim O'Leary

Tim O'Leary is a security consultant, trainer and technician who has also been writing articles on all areas of locksmithing & physical security for many years.