Timing is an important element in access control systems. When a valid credential or code is entered into the reader or keypad, the door unlocks for a predetermined period of time, then relocks. Frequently the door which is being controlled is also monitored as part of the premises' perimeter alarm. If an unauthorized opening occurs, the event may be recorded, reported, or in some way annunciated.
An unauthorized opening is when the door opens without either a credential or code being presented. When the door is opened without authorization, a "Forced Door" alarm is generated by the access control system. A variation on the Forced Door occurs when the door is not closed within the predefined time allowed for an individual to transgress the opening. This condition often referred to a Propped Door.
If the door is opened without credentials or if the system thinks the door is opened before the shunt can occur, an alarm will result. The access controller processes the inputs sent to it. In the real world, sometimes the signals get crossed, and it will be on you to straighten out the problem.
Another situation where timing is critical is when interfacing a door operator with an electric locking device controlled by an access control system. (See Locksmith Ledger April 2005 Towmotor article). In this scenario, the door(s) is (are) being held locked by an electromagnetic lock, and in order for the door operator to open the doors, the electromagnetic lock must be released first.
For a variety of reasons, it may be necessary to augment the existing hardware in order to accomplish the desired system functions.
For both of the situations just described, a sequencer circuit will solve the problem. The word 'sequencer' doesn't really tell you too much by itself. We think of a sequence as the order in which things occur, and there needs to be more than one event in order for them to be sequenced. For applications in access control, there is a sequence of events, and the system controls the timing and sequence of some of them.
The best way to start is to identify the issues and develop a plan of attack. Then implement the equipment needed to execute the plan. The basic building blocks in such alchemy are electronic circuit boards such as timer modules and relays.
Here is a sequencer design which you may find helpful for some future situation. Our sequencer is based on a timer module manufactured by Altronix. This company manufactures a large assortment of power supplies and modules for use by the security, fire, CCTV and access control industries.
Altronix 6062 Multi-Purpose Timer Module
The Model 6062 programmable timer is suitable for many functions that require a timed operation, e.g. access control applications.
The 6062 supports functions such as One Shot, Delayed Release, Delayed Operate and Delayed Pulse. It is also field-selectable for either 12 or 24 VDC operation, so it's easy to stock.
- 12VDC or 24VDC selectable operation.
- Quick and accurate time range adjustment from 1 second to 60 minutes.
- LED indicates relay is energized.
- Form "C" relay contacts are 8 AMP at 120VAC/28VDC.
- Current Draw: Stand-by 3mA, Relay Energized 40mA.
- Triggers via positive DC (+) voltage, dry contact closure, or removal of contact closure.
- Selectable relay activation at the start or end of the timing cycle.
- One (1) second momentary relay activation at the end of the timing cycle (eliminates the need to use two timers for this function).
- Built-in reset feature which cancels timing cycle.
- Repeat (pulser/flasher) mode.
- Snap Trac compatible (order Altronix Model# ST3)
- DIN Rail Mount version available (order Altronix model# DTMR1).
- Lifetime Warranty.
Our sequencer operates within a 24 VDC environment, so we set the timers to 24VDC to keep it all as simple as possible.
The handicapped exit switch is a normally open (N.O.) dry switch closure. The door operator to which we're sending our signal requires a momentary dry closure to trigger the door operator. The word 'momentary' is a very unscientific term, because it is not a precise measurement of time. We needed to present our door operators a contact closure of more than one second in order to achieve consistent operation. We tried a one second pulse and got inconsistent results from different door operators. So we used one of the 6062's built in features, a "Pulse Stretcher."
If you review the programming instructions for the 6062 (download them from the Altronix website, or they come packed with the timer module) and examine the circuit diagram, you can understand how it accomplishes what it does.
The sequence of operation is as follows:
The handicapped button is pressed which triggers Timer #1. Timer #1 actuate Relay #1 (Altronix RBUL) which in turn applies a switch closure across the access controller REX terminals, which tells the system the door opening is authorized and releases the electromagnetic lock on the door.
After Timer #1 cycles, the relay on Timer #1 turns off and the pulse this creates on the trigger terminal on Timer #2 causes Timer #2 to actuate. This presents a switch closure across the input terminals on the door operator, causing the door operator to open the doors. Timer #1 was set to a very short relay activation time because the REX input on the access controller only needed a short pulse to trigger reliably.
Go Forth and Multiply
While we're discussing circuits that solve design problems in the field, here is another quick solution for a common problem. You are installing a REX device on a door which is controlled by an access control system and locked with an electromagnetic locking device. The REX device is equipped with an S.P.D.T relay. The REX terminals on the access controller are isolated; that is, neither of them is common with the electromagnetic lock power supply, so a dedicated pair of wires must be connected to the REX terminals in order for the access controller to operate properly.
Hopefully you have already identified the dilemma in this situation. In order to safely control the door, you should directly cut power to the maglock. To suppress Forced Door alarms, you need to send a signal to the REX terminals. But because of the isolated REX terminals, you need two separate sets of contacts switching when the REX is actuated.
Again Altronix Alchemy is employed by wiring in a RBSN relay. The relay is wired in a fail-safe configuration so that if power is lost or if a wire is broken to the REX device, the electromagnetic lock will unlock.
For more information, contact Altronix, 140 58th Street, Bldg. A, 3 W, Brooklyn, NY 11220. Telephone: 888-258-7669. Web site: www.altronix.com.