More on the Dolph Double Trap Circuit (DDT)

Recently you were introduced to the DDT; a way to configure two hold-up alarm (HUA) devised so that they follow suggested CSAA STA 1 document guidelines.  This circuit allows for HUA device redundancy and the reduction of false alarms.  If either device is accidentally activated a trouble signal is sent to the central station.  If both devices are activated an alarm signal is transmitted.

There was also the suggestion from CSAA that a local indicator be applied for when a single HUA bill trap device is activated.  As promised in the original Tech Talk article If you go to this DDT Circuit link you can download a diagram that will show you how to configure a DDT circuit with local LED annunciators.

Some items to note in this configuration:

  • A bill must be placed in trap to close circuit.
  • Relays are shown in a non-energized position, but will be energized when bill is place in bill trap.
  • R1 Limit Current resistor rating is dependent on LED source.
  • Alarm circuit should be programmed as a NO circuit with a short on alarm. Use the EOL provided or specified by panel manufacturer.
  • You can make this configuration with using 12 VDC DPDT relay modules.
  • If using wireless bill traps then substitute the receiver relays for the Bill Trap contacts in the circuit diagram.
  • LED Trouble annunciators would typically be placed in a visible, yet remote location, such as a manager’s office. Either lighted LED would indicate that the a single bill trap is open and should be restored. Both lighted LED’s would indicate an alarm condition.

CAUTION:  Be careful placing remote indicators or audible devices in such a location as to be observed by non-management personnel.  Steps should be taken not to aggravate a dangerous robbery situation with either visual or audible HUA annunciation.




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About the Author


Bob is currently a Security Sales & Integration "Tech Talk" columnist and a contributing technical writer. Bob installed his first DIY home intercom system at the age of 13, and formally started his technology career as a Navy communication electronics technician during the Vietnam War. He then attended the Milwaukee School of Engineering and went on to complete a Security Management program at Milwaukee Area Technical College. Since 1976, Bob has served in a variety of technical, training and project management positions with organizations such ADT, Rollins, National Guardian, Lockheed Martin, American Alarm Supply, Sonitrol and Ingersoll Rand. Early in his career, Bob started and operated his own alarm dealership. He has also served as treasurer of the Wisconsin Burglar and Fire Alarm Association and on Security Industry Association (SIA) standards committees. Bob also provides media and training consulting to the security industry.

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