Patent GB-1285305-A (GB)
Title: Improvements in or relating to electric circuit arrangements for testing and sorting metal articles
IPC: H03K17/95
Filed: 1971-04-19
Abstract: 1285305 Sorting metal articles SIEMENS AG 19 April 1971 [16 March 1970] 23682/71 Heading G1N [Also in Divisions G4-6] A coin tester incorporates a bridge circuit having on one of its arms two coils L1, L2, whose fields are additive and are arranged so that coins being tested pass between them, on the opposite arm a variable resistor R7 having one or more capacitors C3 in parallel and on the other two arms fixed and variable resistors R2, R3 respectively. The circuit is connected to an A. C. supply and a resistor provided (R5), Fig. 2 (not shown) to vary the supply voltage to the bridge circuit. As shown resistor R6 which has a negative temperature coefficient is in series with coils L1, L2 to give that arm of the bridge the same temperature coefficient as the arm containing the resistor R2. The output from the bridge passes to a differential amplifier V whose digital output passes to sensors W which may be two trigger devices. Preferably the amplifier V is an operational amplifier having feedback (positive or negative) to the non-inverting input and a smoothed output to the sensors W. Depending upon the nature of the coin under test the amplifier V will provide one or two pulses or no pulse. On receiving an acceptable coin (see Fig. 4) the impedance of the bridge arm having the coils L1, L2 falls from an initial value F to a value B and then rises back towards F taking the same line of travel. Since B lies within the acceptance range of the coin tester the bridge circuit causes the amplifier V to emit a pulse while the impedance lies within the acceptance range. This pulse switches a first trigger to move a coin rejecting solenoid from a "reject" to an "accept" position and the coin is therefore accepted. After a given time the solenoid returns to the "reject" position, the given time being sufficiently long to permit the accepted coin to pass the solenoid. An unacceptable coin however creates two pulses because it enters and leaves the acceptance range twice. The second pulse comes before the end of the given time and this pulse causes a second trigger to switch the solenoid back to the "reject" position and so reject the coin. A coin which can create an impedance only reaching the point C produces no pulse to switch the solenoid from its normal "reject" position and is therefore not accepted. The bridge circuit is balanced by the use of the construction shown in Fig. 5 which is an enlargement of part of Fig. 4. A, C are the impedance ranges of unacceptable coins and B the range for accepted coins. The circles of centres P, P<SP>1</SP> represent the sensitivity of the bridge circuit, the centres P, P<SP>1</SP> representing the positions where the bridge is in balance. The centres P, P<SP>1</SP> lie on the perpendicular bisector of 72 which joins the midpoints of the lines A<SP>1</SP> B<SP>1</SP>, B<SP>1</SP> C<SP>1</SP> where A<SP>1</SP>, B<SP>1</SP>, C<SP>1</SP> are the midpoints of the ranges A, B, C. By balancing the bridge to coincide with either P or P<SP>1</SP> only the range B falls within its circle of sensitivity whereas a circle centred at B<SP>1</SP> would include parts of the ranges A, C.