diff --git a/src/flags.ts b/src/flags.ts
index 0046a7fbf..53baf8cbb 100644
--- a/src/flags.ts
+++ b/src/flags.ts
@@ -39,7 +39,11 @@ export type Flag =
    * Added to support user-testing and has the nice side-effect of disabling
    * the dialog for local development so is worth keeping for that use alone.
    */
-  | "noWelcome";
+  | "noWelcome"
+  /**
+   * Simulator. Implementation is very incomplete.
+   */
+  | "simulator";
 
 interface FlagMetadata {
   defaultOnStages: string[];
@@ -52,6 +56,7 @@ const allFlags: FlagMetadata[] = [
   { name: "dndDebug", defaultOnStages: [] },
   { name: "livePreview", defaultOnStages: ["local", "REVIEW"] },
   { name: "noWelcome", defaultOnStages: ["local", "REVIEW"] },
+  { name: "simulator", defaultOnStages: [] },
 ];
 
 type Flags = Record<Flag, boolean>;
diff --git a/src/simulator/RangeSensor.tsx b/src/simulator/RangeSensor.tsx
new file mode 100644
index 000000000..82cde2b9b
--- /dev/null
+++ b/src/simulator/RangeSensor.tsx
@@ -0,0 +1,70 @@
+import { Icon } from "@chakra-ui/icons";
+import {
+  HStack,
+  Slider,
+  SliderFilledTrack,
+  SliderMark,
+  SliderThumb,
+  SliderTrack,
+} from "@chakra-ui/react";
+import { useCallback } from "react";
+import { RiQuestionFill } from "react-icons/ri";
+import { Sensor, sensorIcons } from "./model";
+
+interface RangeSensorProps {
+  value: Sensor;
+  onSensorChange: (id: string, value: number) => void;
+}
+
+const RangeSensor = ({
+  value: { id, min, max, value, unit },
+  onSensorChange,
+}: RangeSensorProps) => {
+  const handleChange = useCallback(
+    (value: number) => {
+      onSensorChange(id, value);
+    },
+    [onSensorChange, id]
+  );
+  const valueText = unit ? `${value} ${unit}` : value.toString();
+  return (
+    <HStack pt={5}>
+      <Icon
+        as={sensorIcons[id] || RiQuestionFill}
+        aria-label={id}
+        color="blimpTeal.400"
+        boxSize="6"
+      />
+      <Slider
+        aria-label={id}
+        value={value}
+        min={min}
+        max={max}
+        onChange={handleChange}
+        my={5}
+      >
+        <SliderTrack>
+          <SliderFilledTrack bgColor="blimpTeal.600" />
+        </SliderTrack>
+        <SliderThumb />
+        <SliderMark value={min} mt="1" fontSize="xs">
+          {min}
+        </SliderMark>
+        <SliderMark value={max} mt="1" ml="-3ch" fontSize="xs">
+          {max}
+        </SliderMark>
+        <SliderMark
+          value={value}
+          textAlign="center"
+          mt="-8"
+          ml={-valueText.length / 2 + "ch"}
+          fontSize="xs"
+        >
+          {valueText}
+        </SliderMark>
+      </Slider>
+    </HStack>
+  );
+};
+
+export default RangeSensor;
diff --git a/src/simulator/Sensors.tsx b/src/simulator/Sensors.tsx
new file mode 100644
index 000000000..a7e521406
--- /dev/null
+++ b/src/simulator/Sensors.tsx
@@ -0,0 +1,31 @@
+import { BoxProps, Stack } from "@chakra-ui/react";
+import { Sensor } from "./model";
+import RangeSensor from "./RangeSensor";
+
+interface SensorsProps extends BoxProps {
+  value: Sensor[];
+  onSensorChange: (id: string, value: number) => void;
+}
+
+const Sensors = ({ value, onSensorChange, ...props }: SensorsProps) => {
+  return (
+    <Stack {...props} height="100%" width="100%" p={5} spacing={3}>
+      {value.map((sensor) => {
+        switch (sensor.type) {
+          case "range":
+            return (
+              <RangeSensor
+                key={sensor.id}
+                value={sensor}
+                onSensorChange={onSensorChange}
+              />
+            );
+          default:
+            return null;
+        }
+      })}
+    </Stack>
+  );
+};
+
+export default Sensors;
diff --git a/src/simulator/Simulator.tsx b/src/simulator/Simulator.tsx
new file mode 100644
index 000000000..c7ce517db
--- /dev/null
+++ b/src/simulator/Simulator.tsx
@@ -0,0 +1,48 @@
+import { AspectRatio, Box, HStack, IconButton, VStack } from "@chakra-ui/react";
+import { useRef } from "react";
+import { RiPlayFill, RiStopFill } from "react-icons/ri";
+import Sensors from "./Sensors";
+import { useSimulator } from "./simulator-hooks";
+
+const Simulator = () => {
+  const ref = useRef<HTMLIFrameElement>(null);
+  const { play, stop, sensors, onSensorChange } = useSimulator(ref);
+  return (
+    <VStack spacing={5}>
+      <Box width="100%">
+        <AspectRatio ratio={191.27 / 155.77} width="100%">
+          <Box
+            ref={ref}
+            as="iframe"
+            // This needs changing before we remove the flag.
+            src="https://stage-python-simulator.microbit.org/simulator.html"
+            title="Simulator"
+            frameBorder="no"
+            scrolling="no"
+          />
+        </AspectRatio>
+        <HStack justifyContent="center">
+          <IconButton
+            variant="solid"
+            onClick={play}
+            icon={<RiPlayFill />}
+            aria-label="Run"
+          />
+          <IconButton
+            variant="outline"
+            onClick={stop}
+            icon={<RiStopFill />}
+            aria-label="Stop"
+          />
+        </HStack>
+      </Box>
+      <Sensors
+        value={sensors}
+        flex="1 1 auto"
+        onSensorChange={onSensorChange}
+      />
+    </VStack>
+  );
+};
+
+export default Simulator;
diff --git a/src/simulator/model.ts b/src/simulator/model.ts
new file mode 100644
index 000000000..37a20a211
--- /dev/null
+++ b/src/simulator/model.ts
@@ -0,0 +1,16 @@
+import { IconType } from "react-icons";
+import { RiSunFill, RiTempHotFill } from "react-icons/ri";
+
+export const sensorIcons: Record<string, IconType> = {
+  temperature: RiTempHotFill,
+  lightLevel: RiSunFill,
+};
+
+export interface Sensor {
+  type: "range";
+  id: string;
+  min: number;
+  max: number;
+  value: number;
+  unit?: string;
+}
diff --git a/src/simulator/simulator-hooks.tsx b/src/simulator/simulator-hooks.tsx
new file mode 100644
index 000000000..047986a19
--- /dev/null
+++ b/src/simulator/simulator-hooks.tsx
@@ -0,0 +1,99 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+import { useFileSystem } from "../fs/fs-hooks";
+import { Sensor } from "./model";
+
+export const useSimulator = (ref: React.RefObject<HTMLIFrameElement>) => {
+  const fs = useFileSystem();
+  const [sensors, setSensors] = useState<Record<string, Sensor>>({});
+  const readyCallbacks = useRef([] as Array<() => void>);
+  useEffect(() => {
+    if (ref.current) {
+      const messageListener = (e: MessageEvent) => {
+        const simulator = ref.current!.contentWindow;
+        if (e.source === simulator) {
+          switch (e.data.kind) {
+            case "ready": {
+              setSensors(
+                Object.fromEntries(
+                  e.data.sensors.map((json: Sensor) => {
+                    return [json.id, json];
+                  })
+                )
+              );
+              while (readyCallbacks.current.length) {
+                readyCallbacks.current.pop()!();
+              }
+              break;
+            }
+            case "serial_output": {
+              // TODO: serial
+              break;
+            }
+          }
+        }
+      };
+      window.addEventListener("message", messageListener);
+      return () => {
+        window.removeEventListener("message", messageListener);
+      };
+    }
+  }, [ref, readyCallbacks]);
+
+  const onSensorChange = useCallback(
+    (id: string, value: number) => {
+      setSensors((sensors) => ({
+        ...sensors,
+        [id]: { ...sensors[id], value },
+      }));
+      const simulator = ref.current!.contentWindow!;
+      simulator.postMessage(
+        {
+          kind: "sensor_set",
+          sensor: id,
+          value,
+        },
+        "*"
+      );
+    },
+    [ref, setSensors]
+  );
+
+  const play = useCallback(async () => {
+    const filesystem: Record<string, Uint8Array> = Object.fromEntries(
+      await Promise.all(
+        fs.project.files.map(async (f) => [
+          f.name,
+          (await fs.read(f.name)).data,
+        ])
+      )
+    );
+    const simulator = ref.current!.contentWindow!;
+    simulator.postMessage(
+      {
+        kind: "flash",
+        filesystem,
+      },
+      "*"
+    );
+  }, [ref, fs]);
+
+  const stop = useCallback(async () => {
+    const simulator = ref.current!.contentWindow!;
+    simulator.postMessage(
+      {
+        kind: "serial_input",
+        // Ctrl-C to interrupt.
+        // A specific message would be useful as probably best to clear display etc. here.
+        data: `\x03`,
+      },
+      "*"
+    );
+  }, [ref]);
+
+  return {
+    play,
+    stop,
+    sensors: Object.values(sensors),
+    onSensorChange,
+  };
+};
diff --git a/src/workbench/Workbench.tsx b/src/workbench/Workbench.tsx
index 3d49a2d36..287df03ad 100644
--- a/src/workbench/Workbench.tsx
+++ b/src/workbench/Workbench.tsx
@@ -16,10 +16,12 @@ import { SizedMode } from "../common/SplitView/SplitView";
 import { ConnectionStatus } from "../device/device";
 import { useConnectionStatus } from "../device/device-hooks";
 import EditorArea from "../editor/EditorArea";
+import { flags } from "../flags";
 import { MAIN_FILE } from "../fs/fs";
 import { useProject } from "../project/project-hooks";
 import ProjectActionBar from "../project/ProjectActionBar";
 import SerialArea from "../serial/SerialArea";
+import Simulator from "../simulator/Simulator";
 import Overlay from "./connect-dialogs/Overlay";
 import SideBar from "./SideBar";
 import { useSelection } from "./use-selection";
@@ -57,6 +59,18 @@ const Workbench = () => {
     useState<SizedMode>("compact");
   const serialSizedMode = connected ? serialStateWhenOpen : "collapsed";
   const [sidebarShown, setSidebarShown] = useState<boolean>(true);
+  const editor = (
+    <Box height="100%" as="section">
+      {selection && fileVersion !== undefined && (
+        <EditorArea
+          key={selection.file + "/" + fileVersion}
+          selection={selection}
+          onSelectedFileChanged={setSelectedFile}
+        />
+      )}
+    </Box>
+  );
+
   return (
     <>
       <Flex className="Workbench">
@@ -99,15 +113,21 @@ const Workbench = () => {
                 mode={serialSizedMode}
               >
                 <SplitViewRemainder>
-                  <Box height="100%" as="section">
-                    {selection && fileVersion !== undefined && (
-                      <EditorArea
-                        key={selection.file + "/" + fileVersion}
-                        selection={selection}
-                        onSelectedFileChanged={setSelectedFile}
-                      />
-                    )}
-                  </Box>
+                  {flags.simulator ? (
+                    <SplitView
+                      direction="row"
+                      minimums={[300, 300]}
+                      height="100%"
+                    >
+                      <SplitViewRemainder>{editor}</SplitViewRemainder>
+                      <SplitViewDivider />
+                      <SplitViewSized>
+                        <Simulator />
+                      </SplitViewSized>
+                    </SplitView>
+                  ) : (
+                    editor
+                  )}
                 </SplitViewRemainder>
                 <SplitViewDivider />
                 <SplitViewSized>