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Circuit Project: Universal Ding-Dong
The circuit shown in Fig. 1 provides a mechanical means of striking two gongs or chimes in sequence — one when the doorbell is pressed, the other when it is released. This it does by briefly activating two solenoids in succession — or even two motors to which suitable hammers are attached. It is a circuit which was rejected by a publisher, for the reason that it was thought to be too complex — which really it is. I had been designing various embodiments of the same idea, and this embodiment was not the most elegant. Having said this, it works perfectly well.
The circuit is unusual from the point of view that it is based on two pulse shorteners, IC1a and IC1b. These are essentially two monostable timers with special arrangements at their inputs. Of critical importance, in these circuits, is that the potential between S1 and R1 should change fairly rapidly when S1 is pressed, and that the trigger inputs of IC1a and IC1b should be suitably biased.
C2 serves to debounce pushbutton switch S1 – however, its value cannot be too high, due to the requirements of the pulse shortener circuit. TR1 and R2 serve as an inverter. IC1a is effectively a negative-edge-triggered monostable timer, so that when pushbutton switch S1 is pressed, IC1a’s output goes “high”, TR2 conducts, and solenoid SOL1 is activated. D1 suppresses back-EMF, which could potentially destroy the IC.
When pushbutton S1 is released, C2 rapidly discharges through R1. IC1b is effectively a positive-edge-triggered monostable timer, so that when IC1b’s output goes “high”, TR3 conducts, activating solenoid SOL2. D2 is again provided to suppress back-EMF. R9 and R10 are not strictly necessary in the circuit, but limit damage in the unlikely event of the failure of TR2 or TR3.
Unless a large battery is used for B1, C1 is needed to provide the “whack” required for solenoids SOL1 and SOL2. If the pulses which activate SOL1 and SOL2 seem to be too long or too short (they are less than a tenth of a second each as shown), the values of R7 and C5, respectively R8 and C6, may be adjusted according to the formula t = 1.1 R C seconds. TR1 is a miniature MOSFET. If an equivalent is required, it may be replaced with the same MOSFET as is used for TR2 and TR3. If TR2 and TR3 are not to be found, rough equivalents may be used, on condition that their gate voltage is at least a quarter below the supply voltage.
Ideally, solenoids SOL1 and SOL2 would be 12V push-action types, or pull-action types which have a thrust pin at the back. However, plain pull-action types should work if they are touching the chimes or gongs when the circuit is at rest (they would then pull back, bounce, and strike). Small DC motors may be used with hammers attached, with suitable series resistors if required. These would likely need longer timing periods for monostable timers IC1a and IC1b.
The circuit may use the (original) bipolar version of the 555 timer IC, or its more recent CMOS equivalents. If a CMOS equivalent is used, standby current is likely to be below 2mA. That is, an AA alkaline battery pack would last about two months on standby. For longer periods, a regulated power supply is recommended. The supply voltage will ideally be 12V, but may be reduced to 9V.
Copyright Rev. Thomas Scarborough