Remotely-actuated biomedical switching circuit using transistors consumes no power in the off position and can be actuated by a single-frequency telemetry pulse to control implanted instrumentation. Silicon controlled rectifiers permit the circuit design which imposes zero drain on supply batteries when not in use

Lee, R. D.

English

NASA Technical Reports Server

FIGURE 2 
April 1969	 Brief 69-10117 
NASA TECH BRIEF 
NASA TeCh Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Remotely-Actuated Biomedical Switch 
.	
.	
Battery 
Switch 
1EE;E.___s._JI : 
	
Ar 
6.5' Diameter
Loop
FIGURE 1 
The problem: 
To design an implantable electronic switch that con-
sumes no power in the off condition and can be actu-
ated by a single-frequency telernetery pulse to turn on 
or off implanted instrumentation. 
The solution: 
Switching circuits using transistors have the disad-
vantage that in their off position they require a quies-
cent-state bias current that imposes a constant drain 
on batteries. The unique electrical characteristics of 
the silicon controlled-rectifiers permitted the design of 
a switching circuit which in the off position imposes 
essentially zero drain on the supply batteries. 
How it's done: 
Figure 1 is a circuit diagram of the actuator for the 
remotely-actuated biomedical switch. The actuator 
generates a short burst of rf signal when the spring-
loaded, momentary-contact switch shorts the capacitor
through the inductance of the antenna. When returned 
to its normal position, the switch reconnects the bat-
tery and allows the capacitor charge to be established 
through the resistor. The operating distance of the 
actuator is determined by the voltage of the battery 
(i.e., the charge stored by the capacitor). Since the 
resonant frequency is determined by the value of the 
capacitor and the inductance of the antenna, the op-
erating frequency can be readily adjusted to fall within 
the range of the antenna tank circuit of the implanted 
electronic switch. 
Figure 2 is a circuit diagram for the remotely-actu-
ated biomedical switch. When the electronic switch is 
initially in the de-energized condition, the base of 01 
is biased off through the ferrite antenna, L 1 , to its 
emitter; SCR 1 and SCR2
 are in the "off" condition and 
amplifier switches 02,'Q, and 04 are also biased in 
their off conditions. As a result, there are no current 
(continued overleaf)
	 - 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000117 2020-03-16T17:55:14+00:00Z
paths existing within the switch network. The biomedi-
cal instrument is inactive, and no battery power is 
dissipated. 
When the actuator radiates a single damped-fre-
quency impulse which is picked up by the ferrite 
antenna, L 1 (appropriately tuned by capacitor C1), 
amplifier switch 01 turns on, allowing capacitors C2 
and C2 to charge up to the battery's potential. The 
charging current through capacitor C 2 and gate of 
SCR, will trigger it on, which allows C 4 to charge to 
the battery potential. The initial current flow through 
SCR 1 anode is extremely large due to the unimpeded 
charging of capacitor C4. When SCR1 is turned on, 04 
is activated and the biomedical instrument is thus sup-
plied with voltage and current from the battery. 
Simultaneous with the SCR, turn-on sequence, the 
charging current of capacitor Ca is actuating the gate 
of SCR.,, allowing its cathode-to-anode circuit to con-
duct. Since the anode current is less than its minimum 
holding current, SCR, turns itself off, but the resulting 
current pulse through SCR2 has developed a voltage 
across resistors R4- and R2 which turns on transistor 
switch 03 and actuates transistor switch 02. When 
transistor switch 02 momentarily saturates, it will 
short-circuit SCR 1 anode-to-cathode connections 
which, under low anode current conditions, would or-
dinarily turn SCR 1 off. However, SCR, remains in 
conduction because the time during which its anode-
to-cathode connection is shorted is not long enough 
to sweep clean all the electrons within its anode-to-
cathode semiconductor layers; this condition is the 
result of the high charging current for capacitor C4 
passing through SCR 1 . Since SCR 1 remains in conduc-
tion, the biomedical instrument remains energized for 
an experiment. 
The biomedical instrument can be de-energized on 
command by recycling the actuator. The ferrite an-
tenna picks up the rf pulse from the actuator which 
saturates amplifier switch 01, charging capacitors C2 
and C; to the battery voltage. Since SCR, is already in 
conduction, its gate circuit is inactive and, therefore, 
the charging current through capacitor C2 has no 
effect. However, the charging current through capaci-
tor C 1 to the gates of SCR., will initiate a current im-
pulse through its anode-to-cathode circuit momen-
tarily saturating amplifier switches Q: and 02, thus 
short-circuiting the anode-to-cathode circuit of SCR1.
As a result, SCR, reverts to its nonconducting state 
because its anode current is now at a much lower 
value with capacitor C4 fully charged. Consequently, 
all active semiconductor devices revert to their off 
conditions; the biomedical instrument is deactivated. 
Notes: 
1. The values of resistors R 7 and R2 are selected so 
that the sum of the currents through them is some-
what greater than the holding current for SCR1. 
2. Transistor switch Q4 has a small voltage drop 
across it when saturated; thus, the biomedical in-
strument is supplied with essentially the voltage of 
the battery pack, usually two 1.35-volt mercury 
cells. If the biomedical instrument package is con-
nected between the positive terminal of the battery 
and point A (Q, R 7 , and R2 can be eliminated), 
it will be supplied a potential equal to the battery 
voltage minus the voltage drop across SCR, (usu-
ally about 0.7 volt). The load current of the bio-
medical instrument package must be greater than 
the holding current of SCR1. 
3. The sensitivity of the electronic switch to rf im-
pulses from the actuator drops off as the third 
power of the distance. Additionally, the receiving 
circuit is quite insensitive, but this lack of sensi-
tivity offers an advantage in that the system is not 
likely to be triggered by extraneous noise signals. 
Ideally, the actuator is used in close proximity to 
the implanted electronic switch; other forms of rf 
activation may provide actuation at long distances. 
Antenna inductance L1 is tuned by varying the 
number of turns on the ferrite cylinder core. 
4. The actuator was designed by Mr. George R. 
Cook of the NASA Ames Research Center. 
5. Documentation for this invention is available 
from:
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10117 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Robert Dick Lee 
(ARC-10105)
. 
Brief 69-10117	 Category 01


Remotely-actuated biomedical switch

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