Lamp modulator provides visible indication of presence and magnitude of an audio signal carrying voice or data. It can be made to reflect signal variations of up to 32 decibels. Lamp life is increased by use of a bypass resistor to prevent filament failure

Zeman, J. R.

English

NASA Technical Reports Server

December 1970	 Brief 70-10700 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the National Technical Information Service, Springfield, 
Virginia 22151. Requests for individual copies or questions relating to the Tech Brief progam may be directed to the Tech-
nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Lamp Modulator Provides Signal Magnitude Indication 
A modulator 'for use with standard incandescent 
lamps provides a visible indication not only of the 
presence but also of the magnitude of an audio signal 
All resistors are 1/4 W5% unless otherwise specified. 
Contacts 1.2.3.10,11. and 12 are omitted on printed 
circuit board. 
Figure 1. Modulator's Circuit; Schematic 
carrying voice or data. The device differs from pre-
vious models in that it can be made to reflect signal 
variations of up to 32 db. Other models operate 
through switch-type devices which apply full voltage
to the lamp, turning it either on or off, and poor or 
zero lamp modulation results. 
The lamp-driver final stage of the modulator op-
erates as follows: The operating point VB (bottoming) 
of transitor Q2 (Fig. 1) is established at approximately 
3.5 V. The amplifier Q-point is biased to operate at 
approximately 2.8 V (or close to VB) (Fig. 2). 
The base of Q2 is anchored to the —0V supply only, 
thus restricting the transistor turn-on point (VB ) to 
only the positive half of the operating signal. Polarized 
B	 Bottoming 
KiQA 
D 
5 
1 
V  VO 
VB VC-Volts	 VCC	 Cutoff	 Vc \1 
Figure 2. Load Line (Left) and Wave Form 
capacitor C4, between the collector and emitter of 
Q2, charges to approximately 18.5 V—the steady-state 
voltage as generated by the keeper-current resistor 
R7. This capacitor must be large enough for the time 
constants to be greater than the total "on" time of 
Q2. Point VB now controls the length of time during 
which C4 may discharge back through Q2, which 
means that the amplitude of the operating signal con-
trols the "on" time. When the control signal turns Q2 
"off," the capacitor recharges to the static conditions. 
In effect, capacitor C4 attempts to integrate the on-
off cycle and equate it to a mean dc voltage. The 
process is proportional until the maximum excursion-
QI reaches Vcc, or the "cutoff" point—is reached 
(Fig. 2). Transistor Qi is a standard amplification 
stage for providing adequate signalling to the lamp-
driver stage. The circuit is insensitive to frequency. 
—24 V
7 
I	 R15 tI 1C1 +C9 CR 1 
I-w 
I	 R2 Ri 
I	 ---• S 1 hr R4 __ C2L R3 Lamp 
I	 R Q2 
I-
;j R6 +C4 - 
I	 R7 
I-R14 I- 4C8 j 
R13 Q3 
I	 —w. R12 
I
-
C6 
-Ru I RiO 
-)l I I C7 I
- 
I	 R9 'C5  C5 
-------
(Continued overleaf) 
in 
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https://ntrs.nasa.gov/search.jsp?R=19700000676 2020-03-17T01:08:16+00:00Z
Another feature of the circuit is that it lengthens 
lamp life. Failure of a lamp filament, is usually at-
tributable to variations in filament resistance, causing 
localized hot spots. Consequent melting of the fila-
ment, or thermal stress at the points of stress, results 
in mechanical failure. The severity of the situation 
varies with the degree to which temperature changes 
from cold to hot and vice versa. By using a bypass 
resistor (R7 in Fig. I) to keep the lamp filament near 
its operating point, the circuit reduces this tempera-
ture change and filament life is increased. 
For example, the basic voltage on the No. 327 lamp 
in the circuit described is maintained at apprcIximately 
7 V, at which the filament is barely visible. The re-
sistance at this level is 350 ohms. With full voltage on 
the lamp, the resistance is approximately 525 ohms. 
Consequently, the filament operates between 60% 
"on" (hot), rather than "off" and cold, and 100% "on" 
(hot).
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Kennedy Space Center 
Code AD-PAT 
Kennedy Space Center, Florida 32899 
Reference: TSP70- 10700 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Royalty-free nonexclusive 
licenses for its commercial use will be granted by 
NASA. Inquiries concerning license rights should be 
made to:
	
j 
Patent Counsel 
Mail Code AD-PAT 
John F. Kennedy Space Center 
Kennedy Space Center, Florida 32899 
Source: J. R. Zeman
Kennedy Space Center

(KSC- 10565)
S . Brief 70-10700	 Category 01


Lamp modulator provides signal magnitude indication

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