Electronic hold circuit

In a hold circuit connectable to a subscriber telephone line, a silicon controlled rectifier is turned on by a user operated momentary pushbutton connecting the positive side of the line to the hold circuit. A comparator circuit sensing the initial value of the loop voltage, turns on a transistor and connects a line seizing network across the subscriber line.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present application is related to copending U.S. application Ser. No. 
444760 assigned to the same assignee. 
FIELD OF THE INVENTION 
This invention relates in general to telephone hold circuits and more 
particularly to an improved electronic hold and hold release circuit 
employing a silicon controlled rectifier. 
BACKGROUND OF THE INVENTION 
In the course of receiving telephone calls, subscribers who have more than 
one local extension frequently find it necessary to shunt (i.e. put to one 
side or hold in abeyance) a call received on an incoming line until a 
desired party is called to the telephone or is transferred from one to 
another local extension. The shunting of such a call is more commonly 
known as "holding" of a call. For this purpose telephone instruments are 
provided with a hold circuit activated by a "hold" key or a "hold" button. 
By manipulating this key or button a subscriber is able to transfer an 
incoming call to a holding circuit instead of to a local telephone 
instrument. This holding circuit is essentially a shunting circuit which 
simulates the electrical characteristics of the subscriber's local 
telephone instrument. This allows the handset of the telephone initiating 
the hold to be replaced "on-hook." The hold will be released when any of 
the extension telephones on the initiating telephone are taken "off-hook." 
Present electronic hold and hold release circuits which employ silicon 
controlled rectifiers (SCR) have problems in reliably holding the line on 
long loops, failing to drop the line when required, or maintaining 
excessive leakage current due to failure to cut off SCR holding current 
effectively. 
Additionally, many present electronic hold circuits require a separate 
power source, usually from the 110 VAC power line, thereby creating a 
possible fire hazard. 
SUMMARY OF THE INVENTION 
The improved electronic hold and hold release circuit of the present 
invention is totally telephone line powered and includes SCR sensitive 
gate latching for operation on short or long telephone loops. 
Additionally, the circuit is varistor protected from lightning surges. The 
circuit will sense various types of high or low DC resistance telephone 
instrument extensions when going "off-hook," sensing a change in the DC 
voltage level in the telephone loop due to the additional current drawn by 
the extension telephone. 
When the hold is activated by momentary contact closure (user function 
button), an automatic timing function starts. If an extension telephone is 
picked up before the circuit times out, the hold will be released. If no 
extension telephone, or the master phone, goes "off-hook" within a 
specified time (usually three to six minutes), the hold condition will 
automatically release. 
In the hold release mode, an artificial load is connected to the telephone 
line to simulate an extension telephone going "off-hook." This is done by 
turning on a high voltage transistor with a resistor connected in parallel 
with the SCR. The load thus applied draws away the current that was 
flowing to the latched SCR, dropping it below its minimum holding current 
value and ensuring shut-off of the entire circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The included drawing schematically shows the improved electronic hold and 
hold release circuit and elements well known and common to a telephone 
instrument. Line current is supplied to the hold circuit from the 
subscriber's line via the tip (T) and ring (R) leads and diode bridge 
network 10. Diode bridge 10 ensures that line current at the proper 
polarity is applied to the telephone transmission circuit (not shown) via 
leads 13 and 14 which are positive and negative, respectively, and to the 
hold circuit via leads 17 and 18 also positive and negative, respectively. 
A hookswitch contact 11 is included on lead 13 and is controlled by a 
typical hookswitch mechanism which is manually operable into operated or 
"off-hook" condition, i.e. when the handset is lifted off the telephone 
instrument, or a non-operated, "on-hook" condition, i.e. when the handset 
is placed on the telephone instrument. 
The improved electronic hold and hold release circuit of the present 
invention includes a first sensing device 61 comprising a comparator or 
other like device having a negative input 64 connected to a voltage 
divider comprising resistors 32 and 36 and a positive input 66 connected 
to a second voltage divider comprising resistors 33 and 37. The output of 
sensing device 61 is connected to the base lead of a PNP transistor 51 via 
a current limiting resistor 34. 
A second sensing device 62 comprising a comparator or other like device has 
a negative input lead 63 connected to an RC network consisting of 
capacitor 42 and resistor 38 and a positive input lead 65 connected to the 
aforementioned voltage divider comprising resistors 33 and 37. The output 
of sensing device 62 is connected to the base lead of an NPN transistor 52 
which when turned on provides a current path from positive lead 17 of the 
hold circuit through a load resistor 31 (e.g. 100 ohms) to the hold 
circuit negative lead 18. Diodes 22 and 24 provide current steering during 
operation of the circuit, and capacitor 41 provides a stable voltage 
reference to leads 66 and 65 of sensing devices 61 and 62, respectively. 
Metal oxide varistor 16 is connected across leads 17 and 18 and provides 
voltage surge protection to the hold circuit. 
A description of the operation of the improved electronic hold and hold 
release circuit will hereinafter be described in detail. It should be 
noted that the included drawing shows all contacts and switches in their 
unoperated conditions. With the telephone instrument operated, hookswitch 
contact 11 makes connecting telephone line current from the tip (T) and 
ring (R) leads to the telephone transmission circuit via leads 13 and 14 
and to the hold circuit via leads 17 and 18. Initially, silicon control 
rectifier (SCR) 21 is in an off state as are transistors 51 and 52, 
thereby effectively disconnecting the hold circuit from the subscriber 
line. 
When the telephone subscriber wishes to place a call on hold, pushbutton 15 
is momentarily depressed, triggering SCR 21 into forward conduction 
connecting the positive potential of line 17 to the hold circuit. 
Resistors 33 and 37 and capacitor 41 charging through resistor 33 reflect 
the initial value of loop voltage and apply this voltage to leads 66 and 
65 of sensing devices 61 and 62, respectively. Capacitor 41 will slowly 
update loop voltages for any fluctuations which may occur during the hold 
condition. 
At this time, lead 66 of sensing device 61 is at a higher potential than 
lead 64 thereby placing device 61 in a low output state and turning 
transistor 51 on. With transistor 51 on, loop current is directed from 
positive line 17 through SCR 21, transistor 51, and LED 23 (which turns on 
indicating that the hold circuit is activated) and resistor 35 to the 
negative line 18. This effectively seizes the subscriber's line allowing 
the telephone instrument to be placed "on-hook" and disconnecting the 
transmission circuit. Capacitor 41 follows and charges to the increased 
loop voltage caused by the disconnection of the transmission circuit and 
applies the change in loop voltage to sensing device 61. 
Lead 65 of sensing device 62 is also at a higher potential than lead 63 
thereby outputting a low, turning transistor 52 off. 
When an extension telephone or the master phone is taken "off-hook," 
negative lead 64 of sensing device 61 senses the drop in line voltage and 
becomes more negative in respect to the voltage referenced by capacitor 
41. The output of sensing device 61 thereby goes high turning off 
transistor 51. The negative potential of line 18 is applied to lead 63 of 
sensing device 62 via diode 22 and line 13 making lead 63 more negative 
than the voltage applied to lead 65 by capacitor 41. The output of sensing 
device 62 also goes high turning transistor 52 on. Resistor 31 is then 
connected in parallel with SCR 21 thereby lowering the current flowing 
through SCR 21 to a level well below the SCR holding current value. As the 
SCR holding current drops, SCR 21 turns off disconnecting the hold circuit 
from the line. 
In order to prevent the hold circuit from being activated for long periods 
of time an automatic hold drop is included. When SCR 21 is triggered 
turning on transistor 51 as discussed above, capacitor 42 charges to 
negative potential through resistor 38. The RC time constant of resistor 
38 and capacitor 42 determines a "time-out" period for the automatic 
release of the hold. When capacitor 42 has charged for approximately one 
time constant, lead 63 of sensing device 62 becomes more negative than 
lead 65, which is referenced to loop voltage by capacitor 41. The output 
of sensing device 62 therefore goes high turning transistor 52 on. When 
transistor 52 turns on, load resistor 31 is connected in parallel with SCR 
21 providing simulation of an extension telephone going "off-hook." As the 
current through SCR 21 decreases the subsequent voltage drop is sensed by 
resistors 32 and 36 and applied to lead 64 of sensing device 61. The 
output of device 61 then goes high turning off transistor 51. To ensure 
positive cutoff of all current to the hold circuit, lead 63 of device 62 
is connected to the collector side of transistor 51 via diode 22 and line 
13 thereby connecting lead 63 to the negative potential of line 18 
ensuring that lead 63 remains more negative in respect to lead 65. Load 
resistor 31 draws away SCR current until SCR 21 falls below a minimum SCR 
holding value turning off SCR 21 and loop current to the hold circuit. 
It will be obvious to those skilled in the art that the time in which the 
hold circuit times out may be adjusted by substituting different values 
for resistor 38 and capacitor 42. 
Although the best mode contemplated for carrying out the present invention 
has been herein shown and described, it will be apparent that 
modifications and variations may be made without departing from what is 
regarded as the subject matter of the invention.