A 74121 (One-shot) worked well as a reset to a counter device. There is an enable line using a 7400 NAND gate to allow or disallow the trigger to pass.

While duplicating the circuit, I found that all I have at the moment is a 74LS00. Are the outputs from the low-power Schottky devices compatible with a non-LS input?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Either use a ground-grid or a ground plane. \$\endgroup\$ Feb 10, 2019 at 2:19
  • \$\begingroup\$ Why not use an inverter to A2 and B instead? \$\endgroup\$ Feb 10, 2019 at 7:03
  • \$\begingroup\$ @SunnyskyguyEE75 - A2 and B are tied to 5 VDC, The truth table for the 74121 allows A1 to be a trigger when A2 and B are at a high level. \$\endgroup\$ Feb 10, 2019 at 16:47
  • \$\begingroup\$ Note how you tie them to Vcc with a 10k R for safety in case 5V overshoots on startup or spikes on disconnect. \$\endgroup\$ Feb 10, 2019 at 17:25
  • \$\begingroup\$ With an inverter on A1 or A2 then that input AND B are both needed high to trigger, normally A! to enable AND B to trigger ^ with hysteresis. \$\endgroup\$ Feb 10, 2019 at 17:37

2 Answers 2


For the circuit as shown, yes. According to my late-70's databook, 74LS parts have an output current of 400\$\mu\$A at 2.5V out, while 7400-series logic has an input current of 40\$\mu\$A (the similar rating for 74LS logic is 20\$\mu\$A). You certainly can't drive as many 74xx inputs from a 74LSxx output as you can from a 74xx output, but just as certainly you can drive one.

I was going to check the fan-out ratings of 74LSxx logic, but as I was turning to the paper book I realized that it didn't have a search bar, and desisted.

  • 1
    \$\begingroup\$ Agreed. 74LS was specifically designed for interface compatibility. Fanout is the only issue. \$\endgroup\$ Feb 10, 2019 at 0:59
  • \$\begingroup\$ Fan out is trivial, cap decoupling an R pull-ups are often overlooked \$\endgroup\$ Feb 10, 2019 at 21:14

That will work as you have shown. But of course, there are many other options .

74’ family  @Vcc=4.75(min)
  Vil = 0.4V @ 1.6mA  ,0.8V max
  Vih = 2.4V@40 μA, Iol = 16mA @ Vol = 0.2V typ, 0.4V max 0.2 0.4V
  Ioh = –40uA @ Voh = 2.4V min, 3.4V typ
  Iol = -1.6mA @ Vol =0.4V  

- means Rol=0.4/1.6mA=250Ω equivalent Rce of open collector saturated.

  • means Rln”0”=(4.75-0.4V)/1.6mA= 2.7kΩ to Vcc equiv. Input resistance for a logic “0” or 0.64mW.
  • means Rin”1”=(5.25-2.4V)/40μA=71k to Vcc @2.8mW but works floating @2V with less margin or 10k pullup to 5V for more margin.
  • yet Ios = output Logic “1” short to gnd = 55mA max from Rc limiting on an emitter follower

This is the classic behaviour of TTL which uses

The 74121 already has 3 inputs two low and one hi, you seem to need two hi. 74LS output can drive 5x 74xx inputs or 10x 74LS inputs.

TTL was always active low as it was the lowest power and lowest driver impedance mode for NPN driver switches, and this resulted is the fastest RC=T slew rate. Each and every TTL family had a fan out drive load of 10 units and most inputs were 1 unit in the same family. Thus the current drive was always asymmetric with the actual threshold voltage always being 2 diode or Vbe drops for every family, 74’,74F’,74S’,74LS’, and 74L’. Power consumption, current levels and speed are the tradeoffs for these families yet all of them used the same output/input current ratio per “10 units” of “fanout” drive current for each family. Thus to use one family with another, there were standard conversion factors just like currency between countries for current yet all have almost identical input voltage threshold, by design.

When CMOS came along since they used almost no DC input current, and were forced to have matched symmetrical Nch/Pch drivers to minimize and limit shoot thru both complementary drivers were active. Even though they make TTL compatible thresholds for CMOS with the “T” in 74HCT’ , one can also use a pullup R to Vcc from TTL to std. threshold 5V CMOS.

TTL 74 and 74S series had very sensitive inputs to overvoltage on B-E reverse emitter inputs and the inputs will breakdown if they exceeded 5.5V. This was later improved using Schottky diode inputs with Schottky transistors.

Thus a 10K resistor ought to be used for unused inputs only if a logic “1” is needed.

They can be shared to many unused inputs.


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